Assuming it's the form where at least one of the people die, I couldn't. I would watch if it was just fighting. Although the forced fighting thing would presumably also not exist in modern times...that would have bothered me too.
I voted "no, and I'd hate everyone who did" - hate not being entirely accurate. More accurate would be "I'd not go, and I'd think less of anyone who did". It definitely wouldn't fall under "I'd not go, but it wouldn't bother me if others did." Obtaining entertainment from a likelihood of someone else being killed, and the opportunity to watch, strikes me in a bad way. I'm not fond of fight-type sports anyway, but at least in their currently accepted incarnations there is at least some (debatable) emphasis on the safety of the participants.
I know that the poll stated specifically that people were doing it of their own volition (so I based my answer on that assumption), but in practice it's rarely that simple. I'm sure we can all come up with a number of tragic circumstances where people would "volunteer" because they believed they had no real alternatives.