Legal Question in Criminal Law in Oregon

If it reasonably appeared that the other person in the ring would imminently kill him or cause great bodily injury, then the defendant would have been acting in self-defense and would be entitled to an acquittal. If not, then he would be guilty of murder -- perhaps even in the first degree -- or at least manslaughter.

Self-defense is only allowed against the person who is threatening you. It does not allow you to kill one person in order to avoid a threat from another. There are many crimes the law would excuse under the circumstances you describe, but homicide is not one of them.