He had seen enough death and blood for many lifetimes. He was done with the Army, done with "saving" people that would only go outside and get killed again. Done with the endless split-second life and death decisions, with a life that was nothing but awkward pauses between duties, with carrying the weight of hundreds he killed because he was good enough and failed, or wasn't good enough to begin with.
So he left the Army, left the continent, and found himself a nice, safe, civilian job, as an emergency surgeon in a hospital that took the cases no other hospital wanted to. He carefully never thinks about that choice, not even when the physically and emotionally genius who is somehow his only friend tries to torture him with the truth.
John doesn’t care. He has seen what real torture looks like.
* * *
He had seen enough drawn-out death for many lifetimes. He has taken upon his shoulders too much grief, and, if he were to be honest with himself, has caused others much grief, too. He was good at what he did because he cared; in the end, patients needed that as much as they needed the pitiful, crapshot therapies that were all he could offer them. But he could no longer stay there and care, and he could not stop caring.
So he left the Hospital, left the continent, and set up for himself a nice, safe, relaxed retirement, as the flatmate and prosthetic heart of a highly functional sociopathic genius who was less empathetic than the criminals he defeated, and was more cut off from life than bodies the ex-Doctor had closed the lids of. He carefully never thinks about their relationship, and he's sure his flatmate doesn't think it relevant enough to any ongoing criminal conspiracy to bother thinking about.
James is fine with the arrangement. He knows what he ends up doing to those that care for him.