I'm convinced that some bullys are taught to be so by their parents.

“Son, when you begin school look for the kid who seems weakest, different, without friends; as soon as you know who he his, ridicule him, throw him to the ground, beat him up every time he looks at you; treat him as the most pathetic shit that ever walked on earth"

That's how you get respect and popularity at school.”

Others just pick up that such behavior can be advantageous.