People bully because of their nature -- they are sadists who get delight by imposing suffering on others. Think of the "droogs" of Alex De Large in A Clockwork Orange: they are his partners in crime when Alex is a psychopathic thug, but once he is reduced to a weak and defenseless character, those "droogs" turn against him for no apparent reason -- except that those "droogs" are horrible people. It is easy to see the writer whose wife Alex has raped or the drunk that Alex beat up for no particular reason taking revenge... the "droogs" do what they do out of their character.
The ideal subject of totalitarian rule is not the convinced Nazi or the dedicated Communist but instead the people for whom the distinction between fact and fiction, true and false, no longer exists -- Hannah Arendt.