-Sexual Deviancy (Necrophilia, for instance in the case of Gary Ridgway, Ted Bunty and Jeffery Dahmer)
-Sociopathy/Psychopathy/Other Anti-Social Personality Disorders (H.H. Holmes, for instance), which tend to cause a lack of remorse
-Child Abuse is a heavy indicator, but not necessarily with the case of certain ones like Dahmer. Substance abuse as well is an issue.
-Lack of development in the prefrontal cortex which controls impulse. Ironically, this also explains why many have bed-wetting problems up past the age of 12.
-Slightly below-average to Average IQ, meaning they have difficultly with problem solving and advanced, long-term reasoning that could help them solve issues
-Tend not to work advanced jobs, typically menial jobs (note that the exception to both of these is HH Holmes, but he is his own bag of worms, though medical professionals)
-Men tend to be serial killers more than women, which makes sense because men are more likely to develop anti-social behavior and have lower IQs, as they have a higher spread of results. However, women who are serial killers either are Black Widows (women who kill their partners), Angels of Death (Nurses or other caregivers that kill their clients), sexual predators, or seeking revenge.
-Serial killers tend to be solo-operatives, and crime bosses, and other groups such as the Mafia or gangs typically have their own classifications