So what makes humans to do things that is considered at evil? I think it comes down to how they are brought up. Whether or not they were abused as a child or how they were lacking of nurturing from their parents or loved ones. But then you going to ask so what caused them to be a psychopath or murderer if they were abused as child? The answer might have to do with hormone imbalances and the impaired social behaviors, and one of the hormone is called Oxytocin
Oxytocin is aka feel good chemical. The neurons of oxytocin needs to be properly nurtured in order to develop. When doing great thing such as receiving love, oxytocin releases and making us feel good. It also being released especially when a mother breastfeeding their baby. Abused child tend to lack in the nurturing aspect growing up. Any animals or humans that are lacking in these receptors for oxytocin tend to act like psychopaths. And as you may know, most of the murderers are psychopaths