Voluntaryism (Fight video included)

[goodsamaritan]

http://www.youtube.com/watch?v=e9r2GAAaiKs (this video was posted in the sporting events forum)

For those who can't watch the video: a highly trained fighter beats the [censored] out of a young, morbidly obese guy who has obviously never fought before in his life. The fight occurs in a ring and the fat guy presumably agreed to the fight and was not randomly jumped on the street. As it turns out, the guy doing the ass kicking is 17 and is slightly younger than the morbidly obese guy.

My question: Is the trained fighter doing anything immoral by participating in this fight and destroying the fat kid?

Even if you don't technically consider what the fighter did to be immoral, I imagine that you must feel at least some sort of disgust at his actions. Where does that disgust come from if not from some sense of morality?