Okay, this is probably the most tenuous story bone I've posted so far, but it feels like there's a lot there. Just waiting below the surface. And it's more of a character motivation than anything else.
So many stories that have doctors who have left the medical profession use the crux that they lost the wrong/too many patients. Have a doctor who left because they saved the wrong person (no revenge fantasies, please, ie. the patient killed or has killed the doctor's family, as I believe that's been done as well). With that conflict between their feelings and the Hippocratic Oath, they don't feel they can continue in the profession.
I'm not sure the patient they saved would really be in the current story line. In fact, I don't think they should be, except as a ghost in the doctor's mind. Their own, personal, Fury/guilt. I'm not even sure this would go to the other theme that they have to do something to save somebody and so overcome their reluctance to practice medicine (also done before).