Tough to watch, but at the same time diffcult to look away.
Headbutting is illegal in MMA. As the guys over at Sports Grid note, this apparently has to do with a combination of front office politicking and disengenuous interruptions in traditional fight pacing. What no one seemed concerned with, however, was that headbutting could result in what happens in this video.
Comes with the territory of beating the shit out of someone for a living, but certainly not something you see everyday.