This article from the Atlantic I read a few weeks ago is pretty fascinating regarding psychopath development in children and how it translates into adulthood (and killers). Psychopathic brains generally cannot recognize fear facial expressions and their pleasure centers are stimulated with reward explicitly but punishments are essentially ignored. People with psychopathic brains have to train themselves into accepting a reward and response system that fits into normal empathetic conventions in order to move through society. https://www.theatlantic.com/magazine/archive/2017/06/when-your-child-is-a-psychopath/524502/