What is a monster? At least three concepts have been proposed: Aristotle thinks a monster to be a "mistake of purpose" in nature; Noël Carroll thinks a monster to be a scientifically impossible being that arouses disgust and fear; Cynthia Freeland thinks a monster to be an evil being. Thus a two-headed calf is an Aristotelian monster; a werewolf a monster on Carroll's definition; and Norman Bates of Hitchcock's Psycho a monster on Freeland's concept. These have no interesting overlaps. My project is to discuss Norman Bates and Mark Lewis (of Michael Powell's Peeping Tom). Bates and Lewis are monsters, but only on Aristotle's concept.