A serial killer is conventionally defined as a person who murders three or more people in a period of over a month, with a “cooling down” time between murders. For a serial killer, the murders must be separate events, which are most often driven by a psychological thrill or pleasure. Serial killers often lack empathy and guilt, and most often become egocentric individuals; these characteristics classify certain serial killers as psychopaths. Serial killers often employ a “mask of sanity” to hide their true psychopathic tendencies and appear normal, even charming.