What can also be a “Instigator” answer? Definition:

“a person who brings about or initiates something.. ” (Google)

You will still have to use the word “fight” (or a synonym of it):

He really likes instigating fights.

I still think belligerent is the best answer, but using “instigator” specifically is focusing on the fact that the person in question likes to initiate the fights, as opposed to someone who reacts to instigations violently. Would you be surprised

if this answer had completely flipped?