I have never been one to watch slasher/serial killer films if I can help it. I understand that people enjoy scaring themselves by watching these types of movies but I have never shared that sentiment. On the one hand the storyline and deranged killers in these films are often so twisted and complex they seem far from reality, yet somehow this sort of crazy appeals to us in a way that causes us to believe it just might be true and therefore leaves us with irrational fears of showers, for example, after the film is over (as was the case with my roommates after watching "Psycho" with me, one even kept a knife in the bathroom for days afterwards).
I suppose these movies seem more real to me than science fiction or zombie thrillers that contain more obviously fantastical creatures that are known not to exist, rather than a normal human whose psychological processing has been interrupted by some traumatic but plausible event that has caused them to respond with a gruesome rampage. Perhaps these films engage with some of our deepest, most twisted fears in order to convince us that such sick individuals are out there and perhaps just a step away with every creak in our homes and rustle in the bushes.