Most people bully because they have low self esteem and feel the need to boost it by belittling and lowering the self esteem of others so that theirs is higher by comparison. Sometimes, a bully comes out of an abusive home and feels the need to abuse others and create pain and suffering for them because they are jealous of their happiness and stable home.
Yet another reason for bullying is the fear that they may be like the person they are bullying and will be themselves, bullied. By bullying the person the fear being like, they reduce the appearance of being like or associated with said person. It also comes from ignorance and fear of something the bully thinks is unacceptable and doesn't understand well.
Bullies often were once victims of bulling themselves and are in fact, sad and angry at their bullies, and take out their anger on what they consider a weak and vulnerable person.
Bulling is a complex thing and it helps no one.