A serial killer is typically defined as a person who murders three or more people, with the murders taking place over more than a month and including a significant period of time between each murder. However, different definitions of serial murder have been used by law enforcement, clinicians, academia, and researchers, and they differ on specific requirements such as the number of murders involved, the types of motivation, and the temporal aspects of the murders. The FBIs Crime Classification Manual places serial killers into three categories: organized, disorganized, and mixed (i.e., offenders who exhibit organized and disorganized characteristics) .

In general, serial killers often lack empathy and guilt, and most often become egocentric individuals; these characteristics classify certain serial killers as psychopaths. They often employ a "mask of sanity" to hide their true psychopathic tendencies and appear normal, even charming. The development of a serial killer involves a combination of biological, social, and psychological factors, which exist together in certain individuals.

It is important to note that while serial killers and mass murderers both commit multiple murders, they fall into different categories of murderers. A serial killer murders three or more people in a period of over a month, with a "cooling down" time between murders, while a mass murderer can kill multiple victims at once with no cooling off period.