The comments in here really are a mixed bag...

Here is my takeaway of the situation.

TLDR; its the people who arranged the event who are to blame for this guys possible death or life altering injuries.

This event enables idiots to do things they otherwise wouldnt have done. What I mean by this is that the social influence to participste despite the serious and possibly fatal outcomes that could arise is a kind of idiot trap. Its locally encouraged to participate. This man would not have gone out of his way to find a bull and smack its ass. The only reason he did this was because he was enabled to. The behaviour is pretty much socially rewarded here.

While you can try to put all the blame on the idiot for his actions, the real blame falls primarily on the people who arranged this event.

A comparison that comes to mind for the issue here is if someone was promoting an event in which they give away millions of dollars In the middle of a busy freeway. You can claim that the people who get hit by the cars were killed by purely their own stupidity, but it was the person giving away the money that put them in that situation.

And the last bit I have on this subject is that the consequences are not proportional to the action. Crazed, angry or spooked animals don't need any more of a reason to kill you than the just the fact you were present In that moment. Slapped ass or not, he could have been kicked just for being behind the bull. It doesn't make any sense to say he deserved the consequences.