Why can child bullies grow into adult bullies?

Child bullies grow into adult bullies because nobody stopped them when they were younger and told them it was wrong. They often start as children who may have been exposed to a violent childhood (e.g., being abused as a kid or watching their parents abuse each other). They then think this is the norm and right so they grow up thinking they should beat up their girlfriends/boyfriends. Because nobody stands up to them, they go though life thinking their behavior is normal. They learn that intimidation works and that it gives them what they want.