Yeah, obviously he's not working as an assassin anymore, but I don't need two Gons, that is, two regular kids only with extraordinary powers, in this story. Thus I wanted Killua to retain something from his earlier life (aside from powers). But maybe you are right and during the series he will continue to "heal" from his abnormal upbringing until he's a weeping mess like a kid his age should be when facing deadly situations and continuous stress. Wouldn't that be great?