Serial Killers are villains who personally killed at least three people or more. They usually have "cooling off" periods in which they won't commit any murder, and are mainly motivated by abnormal psychological gratification.

Common traits of Serial Killers include:

• Varying degrees of mental illness or psychopathy, which may contribute to their homicidal behavior.
• Are motivated either by past traumas or perversions/fetishisms.
• Are highly methodical, always following a set modus operandi and killing all of it's victims in the same fashion.
• Are very cunning and manipulative, often lacking any sort of empathy.
• Will always target a specific type of victim, hardly if ever attacking people outside of his target demographic unless pushed into it by circumstance.
• Will often hide their tracks and remain in the shadows, avoiding calling attention to themselves as much as possible : however they often play "games" with authorities via leaving behind letters, emails or "clues" designed to show their superiority over those tracking them, a classic example of this would be letters left by killers such as Jack the Ripper to taunt police.. this is often a sign of their own psychopathic mindset, many serial-killers will also toy with victims' family or friends, offering to reveal information on the crime or body in exchange for "favors", this is most often found in the psychopathic serial killers, rather than the psychotic ones.
• While many serial-killers are considered psychopathic some are psychotic and thus suffer from extreme delusions, often these types of serial-killers are referred to as the "visionary" type killer - their serial-killing is often highly ritualized, target a victim type based on personal belief of "sin" (or "cleansing"), they are also prone to acts of extreme depravity such as grave-robbing, necrophilia or animal-abuse and mutilation.. these are the serial-killers most often seen in media and while they retain enough sanity to try and hide their crimes many of these extreme examples of psychotic killers are imprisoned in mental wards (or asylums in older settings) rather than jail (psychopathic killers are sent to jail as psychopathy is not seen as a curable condition).

NOTES:

• Monsters, Demons, and other fantastical or supernatural villains can only qualify if they display human-like intelligence and follow patterns and methods similar to human/humanoid serial killers, fantastical creatures with simply high body counts DO NOT QUALIFY.
• Do not include characters from children's shows, family-friendly comedies, thugs, organizations, generic Cataclysmic villains, Assassins or other type of mercenaries, terrorists or any kind of character with empathy.
• Do not add villains already under Murderers to Serial Killers or vice versa. Being a serial killer disqualifies the villain from a simple Murderer. In addition, Serial Killers do NOT necessarily have to be Complete Monsters unless they have the intent on claiming other lives in a gruesome manner.
• Mass Murderers and Spree-Killers are NOT Serial Killers, these types of killers lack the Serial Killer's self-survival drive and they often seek either public infamy, death by cop or see themselves as a one-man army destined to change / destroy society.. as such these kinds of killers are much more akin to terrorists or the severely psychotic rather than Serial Killers.