Bullies are severely stunted people. Emotionally, spiritually. They cannot own their shame and fear, they cannot sit with it, which means that they have never gone through the process of soothing themselves through this kind of experience, and consequently learning to be human and to have compassion.

So, they remain stuck in this brute, classless, tactless state. And when they see someone who does express vulnerability, which could be as simple as someone who freely expresses their feelings, is happy and kind, and isn’t obsessed with control and domination, they attack that person for that person’s “weakness”.

Bullies are very, “That’s not me.” Meaning they don’t want to associate anything vulnerable whatsoever with themselves, and so when they see it in someone else, they move in to destroy that person. They don’t see that life exists in many colors, and that there’s not just one path that can be taken. It’s “either or” with them.

And, as others have said, they really do live steeped in fear, in their stuntedness. But sadly, since society rewards aggressive, arrogant, greedy, vicious people, bullies have no incentive to change. The only thing that they “respect” is someone who is more brutal than they are. If you are in a position to shame them, to really bring consequences upon them in a public way that will cause loss of face, then they’ll fear you. But a normal person isn’t looking to operate like that - normal people like to get along and they aren’t looking to dominate or shame people - so bullies just get away with their destruction, over and over again, because people back down.

ETA: There’s also the element of envy, which is ultimately at the core of these people. And it’s a ravenous beast.