Some are simply wired that way.

I’m not sure that they enjoy it deep down inside, unless they’re psychopaths or sociopaths, or have other deep underlying mental health issue.

It could have to do something with hierachial thinking that is encoded in our biology

Psychopathy. Humans are wired for it. In my opinion its a survival of the fittest scenario combined with lack of conscience. The pleasure may come from a feeling of mastery over powerlessness, possibly experienced in childhood. However not all victims become bullies. Some may be genetically wired for it.