This question was something I asked at a really young age, the first time I set foot on a school bus and right away witnessed the horror of my older brother's world on that bus.
He was bullied and picked on severely and relentlessly.

My therapist told me that children actually can be savages and bully in an effort to gain power and control over other children. However, some of these behaviors come from what they see taking place in their home too, even learning to misbehave for attention.
Often what can happen with children is that if their parents are not involved with them in a constructive way, giving them healthy attention and providing a structure where the child gains pleasure and attention from being productive, they don't learn that and can develop these hurtful behaviors.

Unfortunately, some parents bully their children and hit them and are very bossy. Children are so maleable they actually learn to behave that way as well.