" Definitions vary, but the most useful comes from the National Institute of Justice, which describes it as a “series of two or more murders, committed as separate events, usually . . . by one offender acting alone. The crimes may occur over a period of time, ranging from hours to years. Quite often, the motive is psychological, and the offender’s behavior and the physical evidence observed at the crime scenes will reflect sadistic, sexual overtones. "

― Harold Schechter , Fatal: The Poisonous Life of a Female Serial Killer