why do others find it fun to make fun of others?

I would like to start with a story i have a friend that needs a walker to help her get around one day she is out in her community and there is this kid making fun of how she walks. what is the feelings that are going through here mind? on of anger, sad and scared. so in the long run my Friend is scared to go out. it makes them feel like the have power. why dos some on want that kind of power? Is it that the don't know how there feelings can hurt others ore that the want a friend like her. some on that will listen to them and not care what mistake's that they have made.