Psychopaths only use their reptillian brain. Just like we do except that we also use our emotional brain with gives us such things as empathy witch we evolved so we can live in groups and take care of our young. Reptillians dont care for eachother and are on their own. Thats why rewarding things that are good for the survival of a reptile are things like foos sex and specific for humans power. The ability to hurt other gives a psychopath the feeling of power over the other person. Because if you can hurt someone you have power over them. We also have these tendencies but our empathy is overules these traits. Sometimes when people grow op with no love and get abused they dont develop empathy ans become psychopaths because they have to survive. In severe cases they kill people for revenge and the feelinf of power