So my first and only encounter in a two and a half year fort with a million net worth has been a werewarthog. I made the mistake of embarking somewhere where the only shallow metals appear to be native silver and gold, so my dwarves were primarily hammerdwarves. I realized how brutally insufficient they were when I sent ten of them on the hog, only for one to get the hell mauled out of him in a heartbeat, and two to get bitten, all while they hammered completely uselessly on him with their silver hammers.
Queue incredibly late and under-equipped sword-dwarf squad who comes and makes short work of him with a -bismuth bronze shortsword- One get hurt, but thankfully not bit. His left hand doesn't work anymore.
So I isolated my injured dwarves, and found out two of the hammerdwarves were infected, so I kept them in a little house for a few months barricaded in with fortifications in case I felt like using them for target practice later (dwarves would just stare stupidly at them, so I gave up on that.) They got along fine, but were more than happy to rip up the furniture.
Deciding to make a FB trab down in the caverns, I set them up in a small room with a bridge holding them in so I could unleash them when they turned on whatever happened to be down there.
Realizing I had accidentally made one of them captain of the guard, I decided to punish the other one for the furniture destruction crime that either of them were responsible for. So being unarmed dwarves, and both trained to some level of toughness, he beat the crap out of the other dwarf, leaving him bruised but otherwise alright. Then they went back to behaving.
Queue full moon, sudden flurry of blows as they immediately start tearing each other to shreds after the beating. The captain emerged victorious, lacerating the other hog so badly he bled out. It was over in less than a second real time, I thought he had bled out due to the small cut on his hand he got earlier.
Sorry for the wall of text, but what the hell happened?
TLDR: Werewarthogs suddenly start killing each other after justice beating in dwarf form.
Edit: Game crashed and rolled it back before the beating. They murdered each other again, this time with absolutely no provocation. I'm completely clueless now.