A boxer, wrestler, or football player willfully engages in the sport, despite the risk of which he is well aware.
Animals cannot give their consent. And it is wrong, under any circumstances, to kill for sport. Period. Killing for food or self-defense is one thing. To delight in the bloodshed of any being, human or animal, indicates a deep depravity on the part of the participant. In fact, animal torture and abuse is one of the key features of a serial killer (zoosadism).
Don’t hide behind the tradition argument. Gladiatorial fighting was a great roman tradition, and in its case many gladiators knew what they were doing, and at least had a chance of survival and glory. In fact gladiatorial combat existed into the Christian Era on this one excuse. To say that a prolonged, agonizing death of an animal whose last hours are spent being deliberately angered and maddened is acceptable due to “tradition” is one of the sort cases of cultural relativism one can proffer.