Exactly how does a poor self-esteem make one into a bully?
They pick on people to feel better about themselves.
"You take turns, advise and protect one another, even heal or be healed when the going gets too tough. I know! That's not a game--that's friendship!" ~Shelly Mezzanoble, Confessions of a Part-Time Sorceress: A Girl's Guide to the Dungeons & Dragons Game