I tend to think there are two causes:
Inherent and conditioned
Often, these two go hand in hand.
Inherent is genetic. There are gentic conditions that are passed down at birth. These are unavoidable
Then there are conditioned that are learned through example and environment.
Because of the genetic factor, you will find the environment one is raised in to foster the behaviors because the parent doesn't have any other resources to pull from.
You will find, also, that a lot of people who have become serial killers have had a significant blow to heead. While not proven as causation, it is a very common event and it is known that severe head injuries can change a personality.
I bring up serial killers because a lot of them would torture animals, knowing it was causing pain but would do it anyway. This, to me, illustrates that people do intentionally inflict pain, whether physical or emotional, intentionally. They know they are doing it.
One of the best lines I have heard about narcissists is:
"Do they treat every one the same? No. That means, they know on some level what they are doing and are choosing to do it."
