It takes more than size and athleticism. you have to be just a little crazy to volunteer to get punched in the head and know that every fight could be your last. I'd sooner choose someone from a contact sport that's used to someone trying to cause them bodily harm. Hockey, football, rugby, etc.. At least three prerequisite for brutality is already filled.
Hell, hockey fights have their own code. It used to be where you dropped the gloves, removed your lid, threw down. If someone slipped before you really started throwing, you gave the man a chance to get to his feet. When someone dropped, you stopped throwing... Win or lose you took your lumps and congratulated each on a good fight. Closer to boxing in terms of the honor that's originally intended.