He isn't going to die.
He knows this, because he counted the exact number of ways off this roof and he isn't going to die.
Not now, not here.
Not falling.
So when he jumps, he almost smiles, because he knows what's waiting for him, and it isn't death.
But he doesn't smile, because he knows what's waiting for him, and it isn't them.
John. Mrs. Hudson. Lestrade. Molly.
Before he jumped, he had thought it would feel like flying, but when he does, it doesn't.
Falling, he finds, still feels like falling.
Even though he isn't going to die.