“It was the knife that, a moment later, cut off her scream. And her head.” That’s Robert Bloch’s brief, jokey detailing of the shower scene in his 1959 novel Psycho. Alfred Hitchcock’s film version (brilliantly scripted by Joseph Stefano) not only expanded the murder into a montage masterpiece, it set new rules for the horror genre. Rather, it said the old rules no longer applied. Movies could kill off major characters, turn a nice guy into a psychopath into a victim and especially be both clinically realistic and artistically adventurous in the depiction of violence. Virtually all modern horror movies flow from this exalted source — one of the great studies of derangement.