When 55-year-old Stephen Pasceri walked into a Boston hospital last January and fatally shot Michael Davidson, a 44-year-old heart surgeon who had taken care of Pasceri’s late mother, his futile rage deprived others of a superb physician and changed in an instant the lives of Dr. Davidson’s three young children. They are fortunate to be growing up with their mother in upper-middle-class America, where their trauma will gradually heal.