I don't know, trying to think about WHY Moriarty would commit suicide to keep Sherlock from getting the sniper code, and all these other logical questions, just don't make sense to me. I mean, he is insane, right? Completely off his rocker. So he wouldn't think logically. He also wouldn't have to be on drugs to act like that if he was a true pyschopath and about to kill someone. Have you ever watched Charles Manson interviews? Nuts!
Moriarty starts that scene off talking about how "boring" everything is, everyone, except Sherlock, and now even Sherlock will be boring since he has defeated him. So maybe he wants to kill himself. Again, not logical.
I think it's possible that Sherlock thought of the idea that Molly or someone should check the roof because they were meeting up there and there might be evidence...Lestrade could be in on it too. I mean we are talking about Sherlock.
I do love the idea of a Moriarty brother/twin...and having Andrew Scott come back in some capacity. Either nightmares or as the evil brother (of course he might have to look a little different).