Or maybe they can distinguish movies from real life.I don't get the appeal of this genre either. I can't fathom how one would enjoy watching a movie where they know going in that (nearly the) whole cast will be brutally murdered only for the sake of graphically depicting said killings on the screen. If you get off on watching that sort of thing or find it funny, there's probably something wrong with your head.
I don't know where you get the idea people are getting off on it, but people do enjoy being scared, disgusted and testing the limits of what they can tolerate. There's nothing wrong with that, unless they're actually harming people to do it.