It's fiction, as Izzy says, and writers love their nasty characters. They're fun to work with. It's sort of tough to make them the protagonists--in this case, of course, they took the easy way out and made the protag's victims "worse" in most peoples' eyes, so the audience will identify with the antihero. Not meaning to ignore the question of morality, the slippery slope of revenge, but compare for instance Dexter to Tom Ripley, of the Patricia Highsmith novels of half a century ago. Tom kills perfectly decent people because he wants an easier life. Money, position. That was shocking at the time, because there was so much less general understanding then of narcissism and psychopathology, esp. in the US (her books sold much better in Europe). Tom is actually much more true to life, then and now, than Dexter, who seems to me a product of TV-land's incessant need to up the ante, find a new twist.
