I think paying or encouraging someone to do things that cause severe injury should be a crime. This shit and boxing and American football should not exist.
I am very, very reluctanct to ban anything like that, my default being to err on the side of freedom for consenting adults to do whatever they please.
I'm just surprised that so many people seem to like MMA so much.
(I know it's not the same, I know there are a few important differences, but) it feels to me like bullfighting: I can't understand how someone could enjoy watching _that_.
I think that the laws and social norms we have guard against most of that.
eg, talking about people involved in MMA, either as participants or as spectators: only adults, consenting explicitly to that, and “in full possession of their mental faculties”.
Someone tricking you into taking poison is committing murder.
When problems get hairy, we implement more guards: higher minimum age (eg, those below 21 can't drink, those with less than two year's experience riding motorbikes can't ride the most powerful ones), mandatory delays (eg, being forced to wait X days before euthanasia, abortion, gender reassignment).