There are so many, I know. But if you had to choose one to encapsulate him (and would that one could encapsulate him, and hurl that capsule to the deepest trench in the Pacific) which word would you choose?
I would choose “vicious.”
In the first place, it literally means “characterized by vice,” which he is, in the pettiest and the largest sense of “vice.” And figuratively, it also implies a certain inhuman, animalistic, helpless sense of unthinking, unreasoning anger and hostility to anyone or anything outside of himself. And in the additional sense of “vicious circle,” it implies a certain endlessness, a pointlessness to the aggression–he is vicious just because he’s vicious. No other explanation is possible, or even to be hoped for. He is permanently entwined with vice.
Go ahead, do better than that. In a single word.