He is alive.
The very second he realizes Mycroft's people have miraculously pulled him back from the brink of death he knows this is very, very bad.
Moriarty's people were probably still on high alert. Still working, cogs in the machine with a new figure head as their leader. They are prepared to kill everyone he has come to loves at a moments notice. He can't let that happen.
Lying in the near-sterile room in his brother's home, stitched up and impossibly alive, he knows what he needs to do.
Disappearing would be too easy. It's almost simplistic to find someone who has merely disappeared. If you aren't looking though…
When his brother walks in, pale and gaunt, they don't say much. They never do.
"I need to stay dead." Sherlock states simply, eyes forward. "I will need… assistance maintaining the illusion."
Mycroft's head snaps over to him. "What about Mrs. Hudson? Lestrade? John?" He asks, voice sharp enough to stab through the air.
Sherlock thinks of the trained killers waiting to strike. Thinks of Mrs. Hudson unmoving in a morgue, or Lestrade never smiling fondly at him again, or John's expressive face forever unmoving. He thinks of what them knowing will cost, what it will risk.
He gives a small, pained grin. "they will be safer without me, dear brother."