>>1it's both
heredity and environment both are pretty key comoponents to the makeup of a serial killer
typically it involves the subject intertwining violence/murder with their sexual identity, which creates a paraphilia of sexual sadism
then, for them to get hard, they have to move on from porn/fantasies and actually act out
then after the first murder, they need to do more to cum
frmo a biological standpoint, sex is very important to males as our function is to reproduce and procreate
we want to cum, alot
serial killesr are no exception
etc etc
edmund kempers and interesting one
you can find the FBI symposium on serial murder online