Serial Killers are people that murder at least three people (or, in some extremely rarer cases two people) over a period with "cooling off" periods. They may be obsessed with a certain topic or target a specific group of people and usually kill their victims in similar ways: most serial killers live a seemingly normal life when not engaged in murder. They can also be called serial killers if they use the same method each time. In addition, most (if not all) serial killers are highly intelligent, strategical, slippery, and very good liars and manipulators, as well as charming and seductive now and again. Therefore, they are very difficult to catch, even for police. In many cases, the best way to catch a serial killer is with the help from another serial killer, since they are the only people who know how another serial killer operates.
Note: Do NOT add villains already under Murderers to Serial Killers or vice versa. Being a serial killer disqualifies the villain from a simple "Murderer".