I know most people are good but still there are many people, at any age, who bully or try to hurt others through negative comments, etc.. Why do they do that? I believe the answer is very simple:
So the hurt person is removed from competition or is less of a competitor. By competitor I mean anyone who competes for any resources - money, power, friends, respect, etc..
Now, I want to change the world culture in this respect. Hey abusers, the end results are the same if you do good! Yes that sound nuts but it's true. Let me explain. When you do good for people the good must be bestowed upon you. The goodness can, and does, cause people to to behave in ways to where they are not the competitors they once were.
I know this works like a charm because it has worked for me. I went above and beyond for someone and they thought something was "fishy" because no one behaves like that so they gave me the rewards! The rewards were substantial. That's nuts but it's true. Try it! Spread the world - bullies can be way more effective being nice to everyone!
I'm a high school teacher and have been working with kids for several years.
There's a lot of research on this topic, therapies to help reconcile bullies and bullied people, etc. Usually in kids and teens the bullying behaviour comes from a deep feeling of envy and resentment towards everyone around, that condenses in the most vulnerable person available. Usually it's somehting like "why can he be happy/have a supporting family/live in a nicer place" and not me?", so the answer is to try to spread/share the suffering. Most of the times it's done unconsciously, usually therapy works by helping to identify those feelings.