When Gacy heard an ambulance, he would follow to see what sort of exciting catastrophe was in the making. Part of the reason for many serial killers seeking to become cops is probably due to the intensity of the job. Genetics and physiological factors also contribute to the building of a psychopath. One study in Copenhagen focused on a group of sociopaths who had been adopted as infants. The biological relatives of sociopaths were 4 - 5 times more likely to be sociopathic than the average person.
Environment can make or break the psychopathic personality.
When a psychopath does inherit genetically-based, developmental disabilities, its is usually a stunted development of the higher functions of the brain. Infants and children typically have slower brain wave activity, but it increases as they grow up. Not with psychopaths. Eventually, the brain might mature as the psychopath ages. This may be why most serial killers are under 50.