Episode 3.3, "His Last Vow"

[T.o.b.y]

God, I am in mourning.

Like some of you, I can't believe the Mary conclusion. I liked the episode until John forgave her. So not on... I mean, Sherlock nearly died twice. And John was there, he knows that she even hunted Sherlock down after he left hospital, with a gun in her pocket. Wonder what she planned to do with that, light a cigarette for him?... Yes, she so did not plan to kill him.

No. John was never my true favorite, I liked him, but I was more interested in Sherlock. This episode ruined John for him, however. We see him react to Sherlock's betrayel in The Empty Hearse. That was horrible. He forgave him. Yes. But I do not think trying to kill someone who was not threatening your life (even when he was blackmailing you) is forgivable. Not for him. John did not kill the cabbie because he was evil. He did because the cabbie was about to kill Sherlock. It was a moral decision. Mary's decision was anything but that. Effectively, she also killed Sherlock. I don't know how John can forgive that. I can't. That goes against everything I ever thought about him. I don't think I want him to be Sherlock's friend. He's rotten at it. He allowed Sherlock to ruin his entire life for Mary. Sherlock betrayed his own family, risked to be killed by his own brother, killed somebody, and was almost exiled (where Mycroft told him he'd get killed). And John did not intervene. He did not stand up and tell Sherlock that it was alright. That he did not WANT him to do all that for Mary. Nor did he decide to leave with Sherlock. He shook his hand while standing next to his wife. Just like he stood back after Sherlock killed Magnussen. He did not stand on his side, nor did he try to protect him in any way. He stood back.

 

I don't know if I can ever accept that end. Maybe if he had tried to shield Sherlock. Just one gesture, but to me, this episode is purely about John betraying Sherlock and the effort he makes to protect John's happiness. 

 

Interesting thoughts... Never quite seen it from your angle before. Can't entirely agree, but I do see what you mean.

 

I think it makes sense that John forgives Mary and especially that he doesn't want to know about her past. He doesn't even want to know her real name. Because, as was quite obvious during the last two episodes, he's fallen really deeply in love with Mrs Mary Watson and wants to keep her. Which is kind of understandable.

 

Magnussen summed up the characters quite nicely, didn't he: Mycroft's soft spot is his little brother. Sherlock's is John and John's is Mary. They all make rather insane choices for these people. And on the sidelines, there is Molly still desperately in love with Sherlock (I am sure I am not the only person who expected her engagement to end soon...).

 

If this show has a theme, I think Sherlock named it in A Scandal in Belgravia:

"love is a dangerous disadvantage"

 

[Detective Prince]

 

I was actually okay with Magnussen. He was played just...amazingly. I think it's annoying that he only got one season, one proper episode and that's it. The vault was perfect. I called it as they were walking to the door.

Moriarty returning. I was shocked, I didn't expect it but at the same time, I'm going to have to wait to see how they play it out. I love Andrew Scott's portrayal of him. It's so dangerous and he twists everything around him so well. I can't actually tell if I'm really happy about it or not yet.

I won't go into Mycroft in this post or it'll end up being an essay...

Mary. Now it'll be an essay. I want her gone now. I thought she'd go and then it was all "ASSASSIN." And I thought "She's definitely gone." but then John forgave her and now I just want her out of the series. Absolutely no way you'd ever forgive someone for shooting your best friend, baby or not. Life saving or not. All the lies and deceit. I'm somewhat hoping she'll be a target for Moriarty.

 

Oh I don't know, I think there are many times that John would have liked to have shot Sherlock, but no way he could do it himself.  So maybe he see is at some sort of just desserts.

 

Admittedly, if she had actually killed him I think he would have killed her.

 

 

I get what you mean. But I think it's more the lies, plus I don't know the time between the first confrontation about it and then Christmas. I'm assuming it was more than a month or so but it's hard to tell. Forgiveness like that would take ages, in my opinion anyway.

 

[Zain]

 

God, I am in mourning.

Like some of you, I can't believe the Mary conclusion. I liked the episode until John forgave her. So not on... I mean, Sherlock nearly died twice. And John was there, he knows that she even hunted Sherlock down after he left hospital, with a gun in her pocket. Wonder what she planned to do with that, light a cigarette for him?... Yes, she so did not plan to kill him.

No. John was never my true favorite, I liked him, but I was more interested in Sherlock. This episode ruined John for him, however. We see him react to Sherlock's betrayel in The Empty Hearse. That was horrible. He forgave him. Yes. But I do not think trying to kill someone who was not threatening your life (even when he was blackmailing you) is forgivable. Not for him. John did not kill the cabbie because he was evil. He did because the cabbie was about to kill Sherlock. It was a moral decision. Mary's decision was anything but that. Effectively, she also killed Sherlock. I don't know how John can forgive that. I can't. That goes against everything I ever thought about him. I don't think I want him to be Sherlock's friend. He's rotten at it. He allowed Sherlock to ruin his entire life for Mary. Sherlock betrayed his own family, risked to be killed by his own brother, killed somebody, and was almost exiled (where Mycroft told him he'd get killed). And John did not intervene. He did not stand up and tell Sherlock that it was alright. That he did not WANT him to do all that for Mary. Nor did he decide to leave with Sherlock. He shook his hand while standing next to his wife. Just like he stood back after Sherlock killed Magnussen. He did not stand on his side, nor did he try to protect him in any way. He stood back.

 

I don't know if I can ever accept that end. Maybe if he had tried to shield Sherlock. Just one gesture, but to me, this episode is purely about John betraying Sherlock and the effort he makes to protect John's happiness. 

 

Yes, and for what? A woman who had been lying to him since day one? If I were John, I'd have been wondering if she set her sights on me deliberately. You're right, that was so not on. 

 

 

 

[snip]

 

I'm also wondering how he managed to keep her out of his pants for a month. She was sleeping in his bed, wearing his shirt and nothing else around the house, and they were intimate enough for her to be in the bathroom while he was in the tub. And her efforts to find a man to take home at the wedding show she's a woman who likes a good time. How, exactly, did he evade her advances? I actually would have loved some flashbacks showing him making up lots of excuses and dancing away from her wandering hands, that would have been quite funny. 

 

 

Possibly because he was "always at work" - remember Watson hadn't seen him for a month.

 

 

Maybe, but I don't know. I mean, you've been dating someone for a month or less, and you're already sleeping over in their bed and rearranging their kitchen? How does it move that fast, if you haven't even had time to have sex once because he's always working? I guess it could all be blamed on the fact that apparently she was an opportunist who was just using him. As long as he was letting her get close -staying over, moving in, whatever - and she thought she might get some social elevation out of it, she didn't care if they actually got to be physical.  She didn't even appear to notice how lackluster his kisses were, though it could be argued that he was clamming up in front of an audience and was more physical in private. And he did seem comfortable enough with her on his lap. 

 

 

 

Yep. So not on...

 

It's actually taken from canon. Holmes once got engaged to a girl without even getting into too much physical proximity. He wooed her with his charming attitude *cough*. It's a bit unbelievable in modern times but I actually could accept it because it was a direct reference. 

[Let's_have_dinner]

My head is a complete jumble because of Mary. I really liked her and didn't want her to be a 'baddie', and whatever excuse she has for doing 'bad' things, shooting Sherlock is not something I can forgive. And I don't really get why Sherlock says she 'saved his life' - he saved his own life by being smart, and then John and the paramedics and the doctors saved his life. And then, Sherlock again, willed himself back to life to save John. And at the hospital when John told Mary Sherlock had pulled through, she was not happy. I can't wrap my head around John forgiving the woman who shot his best friend, lied to him from the beginning and has so many secrets. I know she's carrying his baby, but... I just can't see it happening. Is that to all be swept under the carpet now? "My wife's an assassin with a false identity, but all is forgiven and it's back to domestic bliss"?

 

Really, I would have preferred her to die, as I thought she was going to. I would have been heartbroken, yes, and felt sad for John, but at least it would have been more believable. Now I don't know what to think or feel. It's all conflicted 

[Karie]

Oh, by the way, did anyone else see that Sherlock's "porn preference" is "normal"? I'm shocked to find he has any sort of porn preference at all. I just assumed the only "porn" he ever bothered with was an image of naked Irene Adler parading around his mind palace. Or maybe Magnussen is just tracking the websites that are visited on his computer when John borrows it? 

[mangopony]

I was late home tonight so I've only just finished watching it and I LOVED IT!!!! It tied up so many things for me that I was concerned about in SOT.

 

 

Firstly, I'm relieved that Sherlock was only playing with Janine for the purposes of getting at CAM. While it was interesting to see him in a "relationship" something didn't quite feel right, so I'm glad he turned out to be as cold and calculating as he did. For me that's the REAL Sherlock - he only seems to care about John and Mrs H (and Mary I believe too). He was so out of character with Janine and that's exactly what I believe he was meant to be like so that it wouldn't sit right.)

 

As for Mary - I'm really pleased with how the episode turned out for her because I really like her still. I think John forgave her so easily for the same reason he forgave Sherlock so easily in TEH - he loves her and can't imagine life without her. Plus, as is pointed out in the very first episode, he thrives on the drama. She didn't kill Magnussen because she too loves John and didn't want him suspected of murder. Whatever she's done in the past, she was the victim in this episode and I was so pleased in how things turned out.

 

Finally....the end! I actually applauded! I LOVED Moriarty and I even said to my boyfriend at the end of TRF that he wouldn't be dead. I know - no one ever seems to die and that's ridiculous but it is escapism after all. And boy - how much better than real life it actually is!

 

 

Like most people I wish we didn't have to wait too long for the next series but I have the entire Benedict Cumberbatch back catalogue in my rental list from Love Film (other movie rental services are available) so I will while away the time watching and rewatching every episode of Sherlock (series three box set out 20th Jan!!) and indulge my Benny C cravings through other means too!

 

Probably too buzzing to sleep much tonight but I'm so satisfied with tonight's episode. Last week I was an emotional wreck. Tonight I simply applaud the genius that was tonight's episode.

 

Best. One. Yet!!!

 

 

 

[Slithytove]

No, no, no! Mary shot Sherlock and John forgave her! What? What? Ok, so Shetlock forgave het, but where is your loyalty, John? She didn't shoot to wound, she nearly killed him but he fought his way back from the dead. Get custody of the kid, John, and then kick her out. How can you love someone who sent your best friend to the brink of death?

 

But I can almost forgive the writers because of the ending.....Please let Moriarty be alive. I don't care if it is a bit daft that people come back from the dead, I love Jim. This series has been wonderful but the one thing lacking has been Moriarty's fabulously over the top madness. Come home, Jim, all is forgiven.

 

I've got to watch this episode a few more times before I can think straight. Watching a new episode is like eating a whole box of chocolates in one gulp - fantastically rich and overwhelming. The different flavours need to be savoured slowly....

[Ruthyone]

Wow!!! I was completely blown away.........what an episode!!!

 

Mary being an assassin of some sorts, CAM as a man with an almighty brain, poor John Watson with all the emotional turmoil, Sherlock being gorgeous and drugging the family and then blowning CAM's big brain out..............and then Moriarty returning.

Wowweeeeeeeeeeeeeeeeeeeeeeeeeeeeeee!!!!!!!

  P.S. I will try and calm down now :brit:

Edited January 12, 2014 by Undead Medic
Excessive font size

[Karie]

 

 

God, I am in mourning.

Like some of you, I can't believe the Mary conclusion. I liked the episode until John forgave her. So not on... I mean, Sherlock nearly died twice. And John was there, he knows that she even hunted Sherlock down after he left hospital, with a gun in her pocket. Wonder what she planned to do with that, light a cigarette for him?... Yes, she so did not plan to kill him.

No. John was never my true favorite, I liked him, but I was more interested in Sherlock. This episode ruined John for him, however. We see him react to Sherlock's betrayel in The Empty Hearse. That was horrible. He forgave him. Yes. But I do not think trying to kill someone who was not threatening your life (even when he was blackmailing you) is forgivable. Not for him. John did not kill the cabbie because he was evil. He did because the cabbie was about to kill Sherlock. It was a moral decision. Mary's decision was anything but that. Effectively, she also killed Sherlock. I don't know how John can forgive that. I can't. That goes against everything I ever thought about him. I don't think I want him to be Sherlock's friend. He's rotten at it. He allowed Sherlock to ruin his entire life for Mary. Sherlock betrayed his own family, risked to be killed by his own brother, killed somebody, and was almost exiled (where Mycroft told him he'd get killed). And John did not intervene. He did not stand up and tell Sherlock that it was alright. That he did not WANT him to do all that for Mary. Nor did he decide to leave with Sherlock. He shook his hand while standing next to his wife. Just like he stood back after Sherlock killed Magnussen. He did not stand on his side, nor did he try to protect him in any way. He stood back.

 

I don't know if I can ever accept that end. Maybe if he had tried to shield Sherlock. Just one gesture, but to me, this episode is purely about John betraying Sherlock and the effort he makes to protect John's happiness. 

 

Yes, and for what? A woman who had been lying to him since day one? If I were John, I'd have been wondering if she set her sights on me deliberately. You're right, that was so not on. 

 

 

 

[snip]

 

I'm also wondering how he managed to keep her out of his pants for a month. She was sleeping in his bed, wearing his shirt and nothing else around the house, and they were intimate enough for her to be in the bathroom while he was in the tub. And her efforts to find a man to take home at the wedding show she's a woman who likes a good time. How, exactly, did he evade her advances? I actually would have loved some flashbacks showing him making up lots of excuses and dancing away from her wandering hands, that would have been quite funny. 

 

 

Possibly because he was "always at work" - remember Watson hadn't seen him for a month.

 

 

Maybe, but I don't know. I mean, you've been dating someone for a month or less, and you're already sleeping over in their bed and rearranging their kitchen? How does it move that fast, if you haven't even had time to have sex once because he's always working? I guess it could all be blamed on the fact that apparently she was an opportunist who was just using him. As long as he was letting her get close -staying over, moving in, whatever - and she thought she might get some social elevation out of it, she didn't care if they actually got to be physical.  She didn't even appear to notice how lackluster his kisses were, though it could be argued that he was clamming up in front of an audience and was more physical in private. And he did seem comfortable enough with her on his lap. 

 

 

 

Yep. So not on...

 

It's actually taken from canon. Holmes once got engaged to a girl without even getting into too much physical proximity. He wooed her with his charming attitude *cough*. It's a bit unbelievable in modern times but I actually could accept it because it was a direct reference. 

 

 

Yes, exactly. It's believable in Victorian times, but not in 2014 with a woman sleeping naked in your bed, sprawling across your lap, and joining you as you take your morning bath. It's kind of funny, actually. Sherlock Holmes... so good at c**k blocking, he even does it to himself! 

 

My head is a complete jumble because of Mary. I really liked her and didn't want her to be a 'baddie', and whatever excuse she has for doing 'bad' things, shooting Sherlock is not something I can forgive. And I don't really get why Sherlock says she 'saved his life' - he saved his own life by being smart, and then John and the paramedics and the doctors saved his life. And then, Sherlock again, willed himself back to life to save John. And at the hospital when John told Mary Sherlock had pulled through, she was not happy. I can't wrap my head around John forgiving the woman who shot his best friend, lied to him from the beginning and has so many secrets. I know she's carrying his baby, but... I just can't see it happening. Is that to all be swept under the carpet now? "My wife's an assassin with a false identity, but all is forgiven and it's back to domestic bliss"?

 

Really, I would have preferred her to die, as I thought she was going to. I would have been heartbroken, yes, and felt sad for John, but at least it would have been more believable. Now I don't know what to think or feel. It's all conflicted 

 

Right there with you, dear heart, every step of the way. 

[Let's_have_dinner]

I've got to watch this episode a few more times before I can think straight. Watching a new episode is like eating a whole box of chocolates in one gulp - fantastically rich and overwhelming. The different flavours need to be savoured slowly....

 

Fantastic analogy! You're quite right! 

[Zain]

 

Interesting thoughts... Never quite seen it from your angle before. Can't entirely agree, but I do see what you mean.

 

 

 

I think it makes sense that John forgives Mary and especially that he doesn't want to know about her past. He doesn't even want to know her real name. Because, as was quite obvious during the last two episodes, he's fallen really deeply in love with Mrs Mary Watson and wants to keep her. Which is kind of understandable.

 

Magnussen summed up the characters quite nicely, didn't he: Mycroft's soft spot is his little brother. Sherlock's is John and John's is Mary. They all make rather insane choices for these people. And on the sidelines, there is Molly still desperately in love with Sherlock (I am sure I am not the only person who expected her engagement to end soon...).

 

If this show has a theme, I think Sherlock named it in A Scandal in Belgravia:

"love is a dangerous disadvantage"

 

 

 

I understand that, and I also believe that John loves Mary. I just don't think his decision is justified. While he might ignore her past, the recent murder attempt on an innocent (Magnussen might be a blackmailer, but he was no threat to her life or that of her child) and two murder attempts on Sherlock clearly happened in the present. I can't believe he can overlook those. Sherlock nearly died on John; he in fact was clinically dead before his heart beat returned. Then Sherlock collapses again in front of John. Mary pointed her gun at John believing him to be Sherlock. Even if he loves her... how can he live with himself knowing that? Even if he wants to keep her, this should be something where morality wins over emotions.

On that note: I don't think John would have forgiven Sherlock if he had murdered Mary without provocation. Or Harry. And even if he had... he would not have continued to live with him. John did not even leave Mary and, I dunno, sleep on Sherlock's couch, until he had enough time to process all of this.

[Karie]

 

 

As for Mary - I'm really pleased with how the episode turned out for her because I really like her still. I think John forgave her so easily for the same reason he forgave Sherlock so easily in TEH - he loves her and can't imagine life without her. Plus, as is pointed out in the very first episode, he thrives on the drama. She didn't kill Magnussen because she too loves John and didn't want him suspected of murder. Whatever she's done in the past, she was the victim in this episode and I was so pleased in how things turned out.

 

Really? That's an interesting take. For me, she was the least victimized of any of the people Magnussen targeted. Every ordinary person is a sinner.  He went after people who were in positions to do something for him, and he used their weaknesses to do it.  Mycroft's love for his brother, Sherlock's love for John, Lady Smallwood's (?) husband's indescretions before they ever met (and her love for him causing her to not want his name to be ruined even after his death --he was dead right? I got that impression.) Mary, however... Mary was the one with something really appalling to hide. She was a murderer who stole a dead woman's name and made a new life for herself, and she just happened to cross paths with a man who would be a danger of blowing her cover.  I see her as no more a victim than I would see Moriarty if he had killed Molly if she had been a witness to his crimes and had been in the position to turn him in. 

[T.o.b.y]

 

Mary was the one with something really appalling to hide. She was a murderer who stole a dead woman's name and made a new life for herself, and she just happened to cross paths with a man who would be a danger of blowing her cover.  I see her as no more a victim than I would see Moriarty if he had killed Molly if she had been a witness to his crimes and had been in the position to turn him in. 

 

Well, I might have gotten it wrong because everything moved so fast, but didn't someone say at some point that Magnussen had been actively threatening Mary?

 

Oh, by the way, can anybody make sense of the bonfire now? I didn't understand that bit.

 

I just wish this show would calm down again. Back off from the superlatives. But I sense that is not going to happen any time soon.

 

[Caya]

Speaking of Molly - oh, the poor girl. Her engagement ended, and now Moriarty's returning (?). She must be in a bad place .

[Karie]

I have to share this, because it's hilarious. Someone on tumblr posted this link, saying,"Please refer to this Catherine Tate skit to understand exactly how John sounds when he stays with Mary."  I have to say, I really agree! 

 

http://www.youtube.com/watch?v=np-L48PwIDA

[Let's_have_dinner]

 

 

Oh, by the way, can anybody make sense of the bonfire now? I didn't understand that bit.

 

 

 

 

Magnussen had John put in the bonfire to test Sherlock to see if John was his 'pressure point'. He saw how far Sherlock would go for him and realised that he was.

[Zain]

 

 

Mary was the one with something really appalling to hide. She was a murderer who stole a dead woman's name and made a new life for herself, and she just happened to cross paths with a man who would be a danger of blowing her cover.  I see her as no more a victim than I would see Moriarty if he had killed Molly if she had been a witness to his crimes and had been in the position to turn him in. 

 

Well, I might have gotten it wrong because everything moved so fast, but didn't someone say at some point that Magnussen had been actively threatening Mary?

 

Oh, by the way, can anybody make sense of the bonfire now? I didn't understand that bit.

 

I just wish this show would calm down again. Back off from the superlatives. But I sense that is not going to happen any time soon.

 

 

 

I think he reminded her of the leverage; but to get to John, then to get to Sherlock, then to get to Mycroft. She was not really his target. Besides John finding out that she had lied to him "big time", there wasn't anything to lose. It was selfish from her side. Nobody she was protecting but her own interests when she hurt all those people in Magnussen's flat and tried to kill Magnussen (and ends up killling Sherlock - he was clinically dead).

 

Magnussen stated that it was to get to Sherlock; to test where his weakness lies. It lies with John. He already knew Mycroft's weakness (maybe he knew about A Scandal in Belgravia), he however did not have a full grasp of Sherlock after his reappearance.

That's how I understood it.

[Ruthyone]

I'm so-so about Mary - does she really love John? Is this another cover-up? Is the father John or maybe Moriarty is?

 

I think if Moriarty returns it's a good thing. If Sherlock can fake his death, why not Moriarty?

[Karie]

 

 

Mary was the one with something really appalling to hide. She was a murderer who stole a dead woman's name and made a new life for herself, and she just happened to cross paths with a man who would be a danger of blowing her cover.  I see her as no more a victim than I would see Moriarty if he had killed Molly if she had been a witness to his crimes and had been in the position to turn him in. 

 

Well, I might have gotten it wrong because everything moved so fast, but didn't someone say at some point that Magnussen had been actively threatening Mary?

 

Oh, by the way, can anybody make sense of the bonfire now? I didn't understand that bit.

 

I just wish this show would calm down again. Back off from the superlatives. But I sense that is not going to happen any time soon.

 

 

 

I'm not sure, I'm about to re-watch, too. I wish I had subtitles!! All I got from the bonfire was that Magnussen says he never intended to let John die, he had people standing by to pull him out if Sherlock wasn't on time. It was merely to see Sherlock's reaction. He had a hunch John was his weakness, and seeing him crawl through fire to save John's life proved it. 

[Let's_have_dinner]

I also feel sorry for Janine. Even if she was using Sherlock as well and managed to make some money, she never did get to have her wicked way with him, poor girl! But then I suppose she did see him in the bath 

[Zain]

I'm so-so about Mary - does she really love John? Is this another cover-up? Is the father John or maybe Moriarty is?

 

I think if Moriarty returns it's a good thing. If Sherlock can fake his death, why not Moriarty?

 

That's a great idea

I suppose we'll have to see the potential for crack fic in this. Will be a nice wait this time. Last time was serious action/suspense/on-the-run/grieving. This time... crack.

 

And yep, Janine got at least to see Sherlock in the bath. I think the fandom would have liked her to share that sight.

[Let's_have_dinner]

When Sherlock says to John, "Give my love to Mary," I have to say, "She doesn't deserve your love Sherlock!!!" 

[Let's_have_dinner]

Right, I must go to bed or I'll never get through tomorrow.

 

One final thought on the whole "Mary has betrayed me horribly and I can't bring myself to forgive her" thing - I'm worried how it will affect my enjoyment of TEH and, especially, TSoT. I have loved the Mary scenes, especially the Mary and Sherlock scenes and their friendship, and now... hm. It's going to take a bit of time to get used to. 

[Ruthyone]

I wonder if Mark Gatiss and Steven Moffat will take the fans' feelings about Mary (the majority are against her) and dispose of the character in the next series?

 

Edited January 13, 2014 by Undead Medic
Text colour difficult to read.

[mangopony]

 

 

 

As for Mary - I'm really pleased with how the episode turned out for her because I really like her still. I think John forgave her so easily for the same reason he forgave Sherlock so easily in TEH - he loves her and can't imagine life without her. Plus, as is pointed out in the very first episode, he thrives on the drama. She didn't kill Magnussen because she too loves John and didn't want him suspected of murder. Whatever she's done in the past, she was the victim in this episode and I was so pleased in how things turned out.

 

Really? That's an interesting take. For me, she was the least victimized of any of the people Magnussen targeted. Every ordinary person is a sinner.  He went after people who were in positions to do something for him, and he used their weaknesses to do it.  Mycroft's love for his brother, Sherlock's love for John, Lady Smallwood's (?) husband's indescretions before they ever met (and her love for him causing her to not want his name to be ruined even after his death --he was dead right? I got that impression.) Mary, however... Mary was the one with something really appalling to hide. She was a murderer who stole a dead woman's name and made a new life for herself, and she just happened to cross paths with a man who would be a danger of blowing her cover.  I see her as no more a victim than I would see Moriarty if he had killed Molly if she had been a witness to his crimes and had been in the position to turn him in. 

 

 

Hmmmm.... We don't really find out much about her background though. Sherlock says she has an English accent now but that it isn't her own (did I read on here/hear in the show that she may be American??) Perhaps she had some career which involved killing (CIA or something similar maybe??) and something went wrong somehow. Perhaps she fled in desperation and took on a new identity to protect herself (a bit like Irene Adler faking her death)??

 

Plus, Sherlock seems to like her so I trust his judgement. AND Sherlock thought that Magnussen was the worst man he'd ever come across so he must have had a really strong hold over her. He knew that John's weakness was Mary, but Mary's weakness was obviously John because she did what she did to protect him - to stop him being convicted of murder she deliberately didn't kill Magnussen and also to stop her past getting to him because she knew it would destroy him.

 

I dunno, I'm just trying to justify still liking her!