I rewatched a fan-video I'll be using for tomorrow's share and it has this brief shot from "Reichenbach". One of the assassins shakes hands with Sherlock after saving his life, and he (the assassin) gets shot for his trouble. And Sherlock gets this look of horror on his face. This man, a complete stranger, is now dead because he got too close to Sherlock. Sherlock must think to himself how like the anti-Midas touch he's becoming: everything he touches turns to dust.
And then on the roof Sherlock realizes that the three people he probably cares most about in the world (Lestrade, Mrs. Hudson, John) are going to die just because of their mere association with him, and that the fourth (Mycroft) is going to have to live with the aftermath. These are not strangers, but unless he pulls this off, they're all going to be just as dead as that stranger.
This is even more devastating under my personal theory of what was going on in Reichenbach: that Sherlock and Mycroft had been using the events of Reichenbach to tease Moriarty out and somehow eliminate him, most likely by killing him. I don't think Sherlock actually intended to "kill" himself until he hears about those three gun-scopes trained on his friends, but he always knew he was playing an extremely dangerous game. My hunch is this started after the pool scene in "The Great Game" and the final showdown between Irene and Mycroft in "Belgravia," showing that Moriarty was a threat to the things both brothers love best. The thing is, if there's even a little bit of truth to those theories, what must it be like for Sherlock in those final moments on the roof, with the sight of the stranger shot through the chest while shaking his hand and thinking that's about to be be his three closest things to friends and this time it will be his fault, because he decided to engage Moriarty?
Just in case the Reichenbach Fall itself wasn't traumatic enough...