They are scared, constantly. Their brain is physically wired that way, search "conservative amygdala". Short version, theirs is enlarged reducing their ability to emotionally regulate while also causing them to perceive threats where none exist and grossly overreacting to those that do.

But like any abuser they don't just accept that they're weak and scared, they manifest it as anger and direct it at those who they see as weaker than they are. It's not even a "hurt you back first" type thing, they derive enjoyment from abusing others. Makes them feel big.

It's quite pathetic, really.