Great looking dempsey man, my favorite fish. What did you feed him most of his life, and although you moved him around alot, what were his water/changes like?
I'll agree that once they get past 5-6 inches they seem to start to mellow out. I have a 6" that I got from a LFS that was rather beat up. Just tattered fins, so it seemed like he was passive. When I put him in my tank it was confirmed he was relatively peaceful. My 4" firemouth flared and chased him for 30 minutes. I was just about ready to separate them when JD decided he had enough, and turned around and called the firemouth's bluff. They lip locked for 5 seconds. That 5 seconds was all it took. The next day the firemouth's lower lip was visibly bruised up badly. After another day or 2 the firemouth's lower lip/jaw fully healed, but the firemouth never tried that again. Anytime JD would roll around he'd swim out of his way.
Gentle giants for sure. There does seem to be the odd homicidal one, but most seem calm. I definitely see where the name comes from though. They're calm but if you back them into a corner with no other option they'll come out swinging just like a boxer and have the power to back it...