A serial killer is most commonly defined as a person who kills three or more people for psychological gratification; reliable sources over the years agree.
Wilson, W.; Hilton, T. (1998). “Modus operandi of female serial killers”. Psychological Reports (Ammons Scientific) 82 (2): 495–498. doi:10.2466/PR0.82.2.495-498. PMID9621726.
Frei, A.; Völlm, B.; Graf, M.; Dittmann, V. (2006). “Female serial killing: Review and case report”. Criminal Behavior and Mental Health (Wiley InterScience) 16 (33): 167–176. doi:10.1002/cbm.615. PMID16838388.
Wilson, W.; Hilton, T. (1998). “Modus operandi of female serial killers”. Psychological Reports (Ammons Scientific) 82 (2): 495–498. doi:10.2466/PR0.82.2.495-498. PMID9621726.
Frei, A.; Völlm, B.; Graf, M.; Dittmann, V. (2006). “Female serial killing: Review and case report”. Criminal Behavior and Mental Health (Wiley InterScience) 16 (33): 167–176. doi:10.1002/cbm.615. PMID16838388.