For some reason, I'm getting the strong suspicion that the parents spent a bit too long trying to indoctrinate their son and not enough time teaching him how to act morally.
You see this with a lot of children were raised in strict religious households - as soon as they get the chance, they do exactly what they weren't told to do, usually because their indoctrination focused on "don't do X, Y, and Z, or you'll go to hell".
I'm reminded of something I came across in a game once, about a boy who was taught that it was sinful to eat the flesh of creatures that possessed souls (because it would damage his own soul). Since souls were invisible and intangible, his priest told him that creatures which could recognize themselves in mirrors possessed souls. So, later on after he grew up, he discovered the horrifying fact that pigs - which he'd butchered and eaten all his life - could seemingly recognize themselves in mirrors. This led him to believe that he was damned, and then he decided that if he was damned anyway, he might as well visit retribution upon all of the townsfolk, who were equally damned (as they'd eaten pork as well).
I wouldn't be very surprised to find that something like that happened here.