I lost a young aro Monday (13"). It happens bro. Sorry for ur loss. Mine is a mystery. When I got home from 24 hour run to Alabama, it was just dead. My vulture cat had left his mark on the corpse, having eaten the stomach area and the aro's lower jaw was eaten. It was an agressive aro, but really mostly at feeding time. I never feed live foods either, just tilapia pieces and pelleted foods which it never wanted. I have never seen it have any issues with a cichlid in the tank so I don't know what happened.
On the upside, the green terror I did surgery on is doing well...