It is a bit unsporting, and most people would rather cheer for a guy with honour. Then again at the same time, Hendo earned lots of respect for that punch on Bisping, I guess it works both ways.
Ultimately though, both fighters know the risks and fight willingly. You fight until it the bell or the ref stops it. It is one of the most violent sports in the world - the very idea of it is to inflict injury onto your opponent to stop him.
Anybody who has been in a real fight can tell you that when your adrenaline is pumping, especially if you have bad blood with your victim, primal instinct will usually take over.