Is a killer born with maladaptive behavior or do they learn it?

A person is born with the potential towards particular tendencies (eg. violence, docility, etc.) but it is his environment and experiences that shape this potential into fruition.

So let's say two individuals are born with a tendency towards a volatile temper. One of them is raised in a loving environment where his emotional, physical, and psychological needs are met, and the other is raised in a place where these needs are a little neglected. Depending on how well they cope with adversity, there is a bigger likelihood that the latter individual may snap under provocation and kill someone, because he wasn't provided with adequate tools with which he can handle his emotional responses to highly stressful situations.

In a previous job I had to be pretty close to killers and hear their stories and their crimes. They fall into three categories, and I believe that two of the categories are learned, the third I think is a distinct possibility they are just born bad.

Some killers kill because of passion - kill girlfriends, wives, boyfriends of cheating spouses, etc. - this is a learned behavior (or a lack of learning how to deal with emotions). The next group are people who are involved with gangs, drugs, etc. these too are people who have learned their behaviors through their life experiences - abused as kids, parents not there, etc.

The final group - and the group that scares me - are the people that kill just for the sake of it. They don't know their victims, they have no motive other than to kill. These I think are born with maladaptive behavior.

I think some are just born with it. How could a person "learn" that it feels incredibly good to kill someone without actually doing it? That's the only way you could know beforehand that you would enjoy it. Though there is the theory that this is learned per ce at a very early age. No one really knows. These are often the worst killers, and the ones who don't want to be but can't control that side of their personality and need it to achieve happiness.