Bully male cichlid fish

[Kichlidae003]

This is a sign of aggression. Some of the time, fights are obvious. I can see the bully beating on its victims. The fighting mostly occurs when there are no witnesses. It's not that fish are smart enough to avoid the eyes of humans when they misbehaving its just that I can't spend all my time looking at my aquarium. So I'm not there to see or hear. He is a 6 " inch. He just kept chasing them to the pin of four corners. I moved him in a separate tank alone for now. He is very dominant behavior. I was in my bed to sleep and then next morning I just found it out that my fish victim is dead. Still not wanting to keep him. Can't give away no one wants him either. I doubt it, if I could trade him to the pet store. What can I do now?