Photo will help dramatically. If it’s just missing an eye, then I’d move it to a small quarantine tank and let it heal up before deciding where to put it. I’ve seen and have kept fish with 1 eye, some happened in my care. As a matter of fact, I also had a Gold Dojo that was keen on munching on eyes for some reason, but left the rest of the fish alone. Anyways, if it’s a literal bullet hole through the head, then my assumption is euthanizing would probably be best, but I’m not 100% sold on this without a photo.