Im going to paste a couple of comments on the video about this song do you agree with them?
This isn't Zangetsu's theme, this is the sound of crushing defeat. You've put all of your effort, done everything you possibly could to deserve victory, but here you are, the loser, and your confused and feel nothing but overwhelming despair. The gap, between you and your opponent, cannot be explained, for if it could have, you would have done everything in your power to conquer it.
This sounds like when your bleeding or being beaten to death but it doesn't hurt anymore... You're just wondering to yourself, "how could this happen?"