Sorry to hear about your loss. I can relate to encountering unexpected aggression. I had a picasso trigger that had never bothered anybody chase a newly added pink-spot goby so bad I had to remove the goby, then kill - nay, murder - a gorgeous flame hawk within 10 minutes of me introducing it to the DT. Turned my back for a few minutes and the deed was done. That picasso is now living in a refugium at my LFS after they took it in for me and it repaid them by removing the eyes from a large passer angel.
I also had a tomini tang go crazy on a potter's angel, keeping the potters pinned in a corner of the tank for a week till I had to remove it as well. Sometimes you just never know, even in a big tank (mine is 260g).
With that said, I've been wanting a regal forever. I tried a Red Sea regal from my LFS a while back but could never get it to eat. Yours sounds like it would do great my tank. If you can't bring yourself to forgive it and want me to take it off your hands, just say the word!