when I was young, I always had this hunch that horror movies have a dirty trick in their sleeves. since I always had better movies to watch, not that many of them have made it into my watch list.
Watching another one recently, I'm convinced that most those movies are made to scare people into religions. they're usually about a creature or demon who hunts humans and it's the duty of a religious organization or person to protect the people from it.
I'm not against watching those kind of movies, but you must be aware of the beliefs they are installing on your subconscious by invoking the element of fear and bypassing your reasoning power. those kind of stories are carefully crafted to suck people into cults.
The negative effect of them on grownups is not that severe and the more educated wouldn't fall victim to the dirty trick. but beware of the scare they leave on children's mental health.