There is a gene that may be responsible, it's called "the warrior gene" (carried by the X chromosome)because it is responsible for aggressive behaviour... a variant of this gene is present in some people and it may also be responsible for depression. Abuse in childhood coupled with this variant of the gene can lead to more aggressive behaviour as an adult and social exclusion or ostracism can also lead to more aggressive behaviour when coupled with this variant of the gene. A healthy and social environment in childhood when this variant of the gene is present does not lead to increased aggressiveness.... but they are still probably psychopaths but just behave better.