The cause of serial killers is not really known for sure but most serial killers become serial killers because of what goes on in their brain.
Serial killers are both born and made and usually when someone is a serial killer later on in life sometimes you can see symptoms and signs during the persons childhood.
But sometimes the child could seem perfectly normal and then later on in years when they get to adulthood they then have the thoughts of killing and then turn out to be serial killers.
Most serial killers kill because of anger, thrill-seeking, financial gain, and attention seeking.
Most times a serial killer cannot be changed or rehabilitated and if a serial killer killed and then went to prison and got back out of the prison on parole then the serial killer would almost 100 percent kill again.
Serial killers also usually have sexual motives as well and do have sex with the victim before killing them as well.
Also psychological gratification is the usual motive but other motives are also what causes it.