People have all different sorts of religious beliefs; I know some now who believe dancing is evil, which I do not. The only one who could tell you why they believe Satan is involved in watching scary movies would be a person who believes that? I think dancing is supposed to be bad because it mimics sexual moves. If I were going to guess about Satan and scary movies, I would think watching evil might encourage evil, "Rosemary's Baby," for example, is not exactly Christian fare? I don't know but don't think too much/often about it as it is not part of my own interests or beliefs?