I'm not struggling with anything. I get where you're coming from -- it's just the wrong place.
It's a death match. Dead = win. That's not an oversimplification of the event, it's the actual victory condition. Anything less is nothing because the entire point of the exercise is to kill your opponent. The fight isn't over until one of the combatants is dead. Celtic can't have "won" beforehand because his opponent was still alive. He gained a significant advantage -- but he never won anything.
Even from the perspective of a sporting match, you don't say that a boxer won a fight if they get knocked out. It doesn't matter how well they did beforehand, they've still lost.
Celtic could have won, absolutely. Should have won, I agree.
But he didn't. In a death match between two opponents, he was the one that died. That's a loss.