Imagine you put a big amount of money on your favorite, he knocks the other guy out and that guy dies afterwards, can you still enjoy the money you won?
Surely you wont be happy about it but these players are professional and aware of the risk for engaging themselves in this kind of sports, plus they're well-paid. There are incidents ending up to this unfortunate situation but its not the aim of boxing and the other sports as well. That's why it requires an athlete to be mentally and physically fit before getting into a fight because it has risk of having a serious injuries.Ethical. MMA had worse and it's also called a sport. You can see blood splattering all around the ring turning the floor from white to red and then broken bones.
I think people just enjoy those types of games. People hurting each other and see the length of how resistant a human being can be.
Even in other sports like basketball. We enjoy it more when the game heats up and they are starting to hurt each other. "Malice in the Palace"
It's human nature and not about ethics anymore.