It's about reasserting control over their own lives (bullies feel helpless which is the irony of it).
"People who find themselves caught up in anomic states -- for instance, soldiers in war or incarcerated inmates -- tend to feel helpless and alienated. They experience a partial or total loss of control. They have been rendered vulnerable, powerless, and defenseless by events and circumstances beyond their influence.
Torture amounts to exerting an absolute and all-pervasive domination of the victim's existence. It is a coping strategy employed by torturers who wish to reassert control over their lives and, thus, to re-establish their mastery and superiority. By subjugating the tortured -- they regain their self-confidence and regulate their sense of self-worth." (Didn't write it)