A serial killer was a person who killed several people over a period of time. (TV: The Name of the Doctor [+]Steven Moffat, Doctor Who series 7 (BBC One, 2013).)
Suzi Costello used the life knife to kill three victims. (TV: Everything Changes) She also murdered her father, as well as several others. (TV: They Keep Killing Suzie [+]Paul Tomalin and Dan McCulloch, Torchwood series 1 (BBC Three, 2006).)
Clarence DeMarco killed fourteen women in Victorian era London. He was sentenced to death, but was saved at the intervention of Madame Vastra in exchange for the information he had on the Doctor. (TV: The Name of the Doctor)
Jack the Ripper was an infamous serial killer who was active in 1888 London. (COMIC: Ripper's Curse)