A cliche of literature is the sympathetic character who does bad things. The moral of the story is usually that as long as you mean well, you basically get a pass.
Showtime's got a new series that turns this cliche on its head. It's Dexter, the tale of a vigilante serial killer. The lead character's main motivation is to quench his deep-seated urge to kill, without getting caught. Not a nice guy.
But objectively speaking, Dexter does a great deal of good. All of his targets are guilty of capital crimes. And he thoroughly investigates the question of their guilt before taking action. I'd gladly trust Dexter's verdict over that of twelve people who failed to avoid jury duty. Frankly, I'd feel safer if there were more (any?) Dexters running around.
Artistically, Dexter works because of the flashbacks. I'm skeptical about the ability of parents to mould their children's characters. But I still get choked up every time Dexter's foster father struggles to give his sociopathic child a moral compass. Intellectually, I doubt it would work. But emotionally, it's very moving to see a father accept that his adopted son will never be a nice person - but still demand that he be a righteous person.