In the movie The Wild Hunt, after a series of incidents occur with Evelyn, the shaman's followers end up kidnapping her and make her wear the skull mask. After she puts on the skull mask she starts freaking out and runs off. They start following her, starting the Wild Hunt, but instead of playing in-game they actually start killing people.
Why did Murtagh and his followers start killing people? Being upset over what happened in a game seems like a rather odd issue to kill people over.
It might seem like a weak explanation, but LARP (Live Action Role-Playing) are taken very seriously, with people completely immersing themselves in their environment and character.
From Murtagh's point of view, he lost Evelyn (i.e. Princess Evlynia to him) and was embarassed by Erik. It seems this was enough of a trigger to send him over the edge.
You are totally correct in pointing out however that whilst this may have been enough to anger him, even to the point of murder, it doesn't remotely explain why all his followers are so eager to get involved. I think that simply has to be chalked down to the fact it made for an entertaining movie.
If you're interested, there is a fascinating thread over at the IMDB, found here, which is a review of the movie from an LARPer.
