I had 3 yellow tail damsels and only 1 was mean. He killed the other 2 and my cardinal. I haven't added any more fish since, but when I do, I'm thinking of flushing the little bugger so he doesn't kill anymore. I didn't know they were so **** mean.
I would suggest, you trade it to another person or your LFS. I would never just flush a living fish down the toilet. I have a 55G full of damsels, I didn't want to try and catch them, I went and got another tank...LOL!
Age is relative, you are only as old as you act....of course, this works in reverse....