I really don't get that. I think it's because I've played so much videogames where I personally "killed" countless "people" while impersonating sometimes less than stellar protagonists. While watching Dexter I have the same feeling as when playing videogames. It's not violent if "killed" is just a bunch of pixels on the screen. In Dexter killed usually have almost no personality only some serious criminal traits. When they are killed I don't feel any more empathy than I feel towards targets in a game. I find shows where characters are properly humanized before getting hurt much more disturbing. And it doesn't even have to be physical harm.