I've read about the killing animal part, that's their initial step to being serial killers.
Animals are often seen as "practice" for killing humans.
Genetics tests are showing that the nervous system of the psychopath is markedly different — they feel less fear and anxiety than normal people. One carefully conducted experiment revealed that "low arousal levels" not only causes impulsiveness and thrill-seeking, but also showed how dense sociopaths are when it comes to changing their behavior. A group of sociopaths and a group of healthy individuals were given a task, which was to learn what lever (out of four) turned on a green light. One lever gave the subject an electric shock. Both groups made the same number of errors, but the healthy group quickly learned to avoid the punishing electric shock, while sociopaths took much longer to do so. This need for higher levels of stimulation makes the psychopath seek dangerous situations.
Is it genetic, hormonal, biological, or cultural conditioning? Do serial killers have any control over their desires? We all experience rage and inappropriate sexual instincts, yet we have some sort of internal cage that keeps our inner monsters locked up. Call it morality or social programming; these internal blockades have long since been trampled down in the psychopathic killer. Not only have they let loose the monster within, they are virtual slaves to its beastly appetites. What sets them apart?