slasher

The slasher film (sometimes referred to as bodycount films and dead teenager movies) is a sub-genre of horror film typically involving a psychopathic killer (sometimes wearing a mask) who stalks and graphically murders a series of adolescent victims in a typically random, unprovoked fashion. 

Psycho is a seminal slasher because throughout the film there is frequent violent murders with use of knives and dangerous weapons. It set a new level of acceptability for violence, deviant behaviour and sexuality in American films. It was also an early example of a slasher film genre.