The Bully often haves a very chaotic life in which they experience problems such as difficulties at school, poor parental relationships, a complex with their own personal appearance or in some cases the own oppressor could have been bullied once before. 

This makes the bully to take out all of its frustrations and insecurities into people which appear to be inoffensive and bland that wont do much on try to defend themselves; also it will give the power and superiority that the bully needs to feel better than anyone else.

Source: www.michellehenry.fr