I want to clear some things up:
1. Ethics is an OBJECTIVE thing. The Bible spells out ethics, it IS where ethics came from.
God is the Father of morality. It is an objective thing.
No, ethics are relative. Even biblical ones.
2. The Bible condemns ALL violence with the exclusion of self defense with life I believe. I don’t know. But I do know Jesus Christ said,
“If someone slaps you, turn the other cheek,”
That is more of a New Testament sort of thing, but that was when someone hit you without agreeing to get hit. I don't recall Jesus saying anything about those that do various combat sports.
3. Consent or simply wanting to do something does not make it ethically correct.
Though it does help a lot .
If I thought it was right or justifiable to kill someone, is that ethical?
It would depend. Is not self defense ethical? How about defense of your family. Let's take a hypothetical. You are married to a lovely young woman, but she is not "saved" yet. You have been discussing religion with her regularly and she appears to be close to changing her mind. Suddenly one night some deranged man attacks you and your family threatening to kill all of you. Your wife, if she dies now, is doomed by your religion to hell. What do you do? Do you turn the other cheek yourself, possibly dying as a result and guaranteeing the death of your family or do you kill in self defense?
In the same way, even when fighters get into the ring to hurt the other and themselves, it is still not ethically justified.
That needs a lot more than a "because I said so". Now you may be able to show that most of them are not justly compensated for the risks that they take. But here is the weird thing. Some of them. Even if they lose, enjoy the experience. They look forward to the competition. How is their competing with others of the same mind unethical?