Well I think you already know what to do. You have 3 choices.
#1 Get rid of the Killer butterfly.
#2 Get rid of the bta.
#3 Nothing, and let the killer fish get the bta.
Personally I would choose #1.
Wow, hard decision, just ask yourself which you are more attached to. I'd check to see if your local LFS would take your cast off for store credit of maybe another local reefer might buy it from you. I am curious how are thing s going this evening? I would do something soon, it is not the best place to keep your bta for too long, it needs to get settled. Maybe we will get a photo, can't wait to see it.