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.
Sure, but OP brought up the term ethics, and that term requires to reflect on boxing and gambling from many different perspectives. If I am different between gambling on chess and boxing, I would always go with chess. Money being put on you also puts pressure onto you. These boxers know what is at stake and try to resist a knockout under any circumstances, maybe at some point not knowing about their own condition but strong enough to hide that fact.
A sport that involves killing like the bull fights in Spain where you can see children yelling for the torero always got me concerned on Youtube. I was once asked to go into an arena in 2010 when it was still allowed, not sure if still is, and place some bets. My friend went there, I didn't.