You can't always predict who will grow up to be a violent criminal or serial killer. These criminals often share certain traits, however. FBI Special Agent Robert K. Ressler developed a list of fourteen traits common among serial killers.
Males represent a large amount of serial killers - more than ninety percent. They are often quite intelligent. Their IQ may even be placed in the "bright normal" range. In their younger days, these criminals may not have done well in school. As an adult, they may have a hard time keeping a job and might end up performing unskilled labor.
They may have had a hard family life. They may be abandoned by their dad, leaving the child rearing to an overbearing mother. Their family life in general is unstable. Also, psychiatric problems, alcoholism or criminal activity may be present in the family history.
Additionally, a serial killer may not have a typical close bond with their parents. They may even come to hate them. Sexual, physical or psychological abuse may have been present in the killer's childhood.
These violent criminals can have a high occurrence of attempted suicide. As children, they may end up spending time in institutions. Psychiatric problems may develop very early in their life.
A curiously strong interest in sadomasochistic pornography, voyeurism or fetishism at a young age can be another strong indicator that something is amiss. A fascination with fire is also common.
Bed wetting among serial killers is quite common. More than half were wetting their bed after 12 years old. Torturing small animals or other sadistic activity is the last key indicator of a potential serial killer. Torturing a spider is one thing, but when a child moves on to cats or dogs, and the torture is not done for an audience, there is major cause for concern.
Serial killers may also feel worthless or inadequate. Poverty or low socioeconomic status and childhood abuse and humiliation can cause these feelings. The crimes committed by these individuals are compensation for these feelings. In their minds, a sense of power comes from terrifying victims and communities and confusing police.
Once caught, some plead "not guilty by reason of insanity". This plea usually hinges on the M'Naghten rule - whether the individual was able to tell right from wrong at the time of their crime. However, some serial killers have no obvious delusions or hallucinations, and exhibit premeditation, which make this plea unsuccessful. The plea does, at least, make for certain evidence to be admissible, like childhood abuse. This is often brought up in hopes a jury will be sympathetic and not recommend the death penalty.
Internal Association of Forensic Science, an article written by FBI Special Agent Robert K. Ressler
"The Serial Killer," Harold Schechter
Wikipedia /Serial Killers