A.N. Forgive the abuse of brackets; they popped up by themselves, I swear.
Disclaimer: nothing is mine (yet).
Mathematics
It's one of these empty days, during the Hiatus (hiatus; because he's definitely going back home – back to John), when he's waiting for information on the current branch of the late Moriarty's network. He can't work without data, after all. Sherlock's mind replays the whole mad game that brought him to this point.
Getting to know he had a fan who sponsored serial killers (and people wondered why he was annoyed by the press...he didn't need more fans, thank you). Five pips and trips down memory lane (and to the basement). The pool and its one-second of pure anguish (because John couldn't be Moriarty), so much so that the panic afterwards tasted like relief. The Woman and discovering Moriarty was still interested in 'the Holmes boys' (Mycroft could keep him, if he liked). And this last round, with all the teasing, feeling the net inexorably close around himself, with each hint at 'pressure points' and 'I O U' and fairy tales... That's when the sleuth realizes something (silly it might be, but he has to entertain himself and drugs are now out of question). Moriarty failed.
No, not because he's dead while Sherlock is alive. Well, he should be dead; at the very least, he was hurt seriously enough that there was no way to extract from him the code to call the snipers off in time. Technically, he could be in a coma somewhere, but Sherlock doubts Jim would like that at all. His own continued survival is a fluke, due entirely to a wild card in a lab coat. After all the times he had hurt her (unwittingly, always) Sherlock wouldn't have been surprised if she offered an extra push, instead of help.
No, Jim failed by his own standards. 'Fairytale villain'? Sure, Sherlock has deleted every fairytale that had been read to him. But the general setting of his final problem was supposed to be one of those 'damned if you do, damned if you don't' situations. Sherlock is reasonably sure of this fact.
A physics problem, with the conflicting forces of self-preservation (with his arrogance to back it up) and whatever selflessness (and affection, can he admit to that?) he possesses meant to lacerate him like Sinis' pine trees (he has deleted most literature, but the Greek myths have so many original murders that he keeps them as reference; criminals should find them inspirational).
But conflicted is the only thing Sherlock wasn't (wouldn't be), even with the certainty of death awaiting on that roof. Scared? Considering the situation, he had some right to be, he reckons. At least, a little. He's still only a human, whatever everyone else might think. Sad? Of course, he was leaving John (could have left him forever), hurting John (sorry, it was inevitable – really). Conflicted? Never.
Because it wasn't a physics problem (the one it was meant to be). It was a mathematics problem. One or four. A choice so easy it wasn't a choice at all. Four: that's what Moriarty didn't realize – what made his whole elaborate scheme ultimately fail at its purpose. Never only three bullets (for the three people in the world that amazingly found Sherlock somehow tolerable) halted – or not – by Sherlock's own fall.
Sherlock wouldn't have survived those deaths. It's not like it sounds (if he were to talk aloud), nothing ridiculous like dying of a broken heart because; a) real hearts can't break; and b) he has no metaphorical heart. To be completely exact, his metaphorical heart (which has a full name, and a degree to boot) would already be gone in such a hypothetical situation. Years past have clearly proved that Sherlock can and will survive without one anyway.
But the whole point of becoming a consultant detective was to exploit his overgrown, relentless brain to help . If he closed the cases for Lestrade (Dimmock, Gregson, Hopkins, Jones...) the crimes would stop. Nobody would get hurt (killed, kidnapped...whatever) anymore. He was very careful to keep cold when faced with any additional victims found between his accepting the case and its resolution, even when he was shunned for it. He always gave his all, and agonizing over it not being enough to save them wouldn't benefit anyone (as he tried to explain to John). It would be counterproductive (he didn't say: wasn't it obvious?). It was better for everyone to just accept new data and use it.
That day, though...he could help it. He could save them all with a single step over the ledge. If he had refused, he would have become as guilty as the snipers, or as Moriarty.
Donovan would have been right. There would have been corpses, and it'd be his fault.
Donovan. Right. Words that did not belong in the same sentence. Just like John and dead. No, these last two were even more (absolutely, totally, ontologically) incompatible. Sherlock so hates grammar mistakes. Making four of them (because Lestrade and Mrs. Hudson, too, can't be in the same sentence with dead...not for twenty years at the very least) with a single act is unthinkable. Surviving the poisoned knowledge that his choice killed Lestrade, Mrs. Hudson – John (he might as well have pulled the trigger himself, in such a speculation's perspective) would have been unimaginable. His mind palace would have crumbled like a house of cards. Extended damage, only option degaussing of the hard drive. Rocket finally exploded. He can't even run the simulation for long. He has to hack Mycroft's cameras afterwards, to make absolutely sure the simulation is a simulation. Thanks to the non-existent god(s), everyone is as well as can be expected.
One or four. Not much of a dilemma, is it? Really, shouldn't villains with literary (however low) aspirations be able to do better?
P.S. For the technologically challenged like me: Degaussing is the process of decreasing or eliminating a remnant magnetic field. For certain forms of computer data storage, however, such as modern hard drives and some tape backup drives, degaussing renders the magnetic media completely unusable and damages the storage system.
For the mythologically curious: a teenager Theseus left his mother (Aethra)'s house in Troezen to go meet his father Aegeus in Athens. Along the way, he met and disposed of a few bandits with interesting modus operandi. Among them was Sinis: he bent two pine trees to the ground, tied up unfortunate travellers to both and then let the pines go. His victims were obviously torn apart. Laborious way of mugging someone, uh? Teen Theseus gave him a taste of his own medicine. Then he proceeded to have sex with Sinis' daughter – they had a child too. Well, such are Greek myths.