A DOP I once worked with, a true "film Geek" and therefore utterly dispassionate, once told me that slasher films work by tapping into the audience's psycho-sexual fears; fear of sex, intimacy, penetration, they work -he claimed- by holding up a mirror to the auditorium and forcing those watching to question their own insecurities and relationships.
Freudian Rubbish. I laughed in his face.
Slasher flicks work because they provide the audience with the two things they truly crave: lashings of blood, and an erection.