Bullies all have several things in common. First, they have major inferiority complexes. There is something about themselves that they feel is lacking, so they often lash out to try to make themselves appear to be big and bad, because they see themselves as small and scared.
Secondly, they generally pick on what they view to be soft targets; smaller victims, homosexuals, people with disabilities. All people they feel will be hindered in their ability to protect themselves.
Thirdly, bullies have smaller male members than non-bullies, if they are males, and female bullies usually possess far less sex appeal than non-bullies.