Are bullies born that way? Or is this a result of a bad upbringing? Or is the nasty behavior the result of sin? I don't really know the answer, and I have yet to pose this question to a Catholic priest. So, at the moment, I'm only just guessing. In church, we tend to hear more about sin, than about narcissism, although Pope Francis has recently spoken out against clerical narcissism.
Actually, I think all three factors, inborn temperament, our formation and sin play a role in whether someone chooses to abuse another. Grace is not distributed equally, so this would have something to do with the way we are born. Our upbringing certainly shapes us. Parents who push people around often have offspring who do the same. So, in a sense, we're looking at a learned behavior.
Sin can also cause spiritual blindness and hardness of heart. It takes a very stony heart to intentionally hurt another.
Narcissistic personality disorder, according to psychological literature, is a fixed condition that can't be changed. However, as Christians, we know that God can do anything. Yet it seems as if it would take a miracle to change some hearts. But we know it's possible, because all things are possible with God.
Flickr photo by Paralog