None of my comments are directed at the actions that happened in Teonn. I was not at the last game, so I don’t have first hand knowledge of what occurred. These are just my general thoughts about Larping and playing evil characters within them.
Things are different in one-off theatre form larps, where PC antagonism is often a major part of the plot. Same with White Wolf Vampire games, where politicking and betrayal are the mainstay of the game.
I think that playing bad guys amongst the heroes can be okay, but can easily lead to bad feelings. The reason is that it is not an even playing field. It is very easy to play the secretly evil character and betray the rest of the PCs at the opportune time. If you play the character as a cheerful and fun character, then there is no reason to think there is anything else to the character until it is too late. Even a quiet and sullen, moody characters can get away with blending in with the good characters until the perfect time to strike.
By picking the right time for the betray, the secretly evil character has a much higher chance of succeeding at screwing things up that the heroes have of foiling them, because they will normally only choose to act when they have the advantage and when the favour is stacked in their favour.
There are a number of factors that lead to trusting others. I think it is partly the OOC assumption that we are all playing heroes with a common goal. This is the one I have the least respect for, because I don’t think that is always the case, and characters can have individual goals which may or may not be aligned with what the majority.
But another factor is the difficulty of telling if someone is lying in a game. Without magic, it is a lot harder than in the real world, because everything that the character is saying is made up, everything is actually a lie. Saying “I am Timothy the Orc. I am a great warrior chieftain!” is a lie, I am actually Dave the human pretending to be an Orc named Timothy. So when the player is making up everything that is considered to be the “Truth” within the setting, it is very easy to make up things that are not the truth within the setting, and the subtle clues that the character is lying are not the same as they would be in a real situation.
Also, it takes quite a bit to stand out as evil among PC groups, because players want to play interesting characters. This often takes the form of a troubled past; a conflicted moral code; emotional baggage of some description or another. So showing signs of “bad” behaviour, what a player may think is a hint at their characters true nature, may not stand out from the pack of troubled heroes.
So, personally, I think unless you are being quite obvious and consistent with clues about the true nature of the character, then a victory by betrayal is actually a rather cheap victory in these sorts of larps. It really is not a surprise that someone can “win” in this sort of thing. Unless they set out to purposely loose, or at less give obvious hints and clues out over the course of the campaign to give a reasonable chance of of being found out, then it would be a surprising for them to lose, in my opinion.
So when someone is “victorious” in this sort of way, I think others can feel hard done by, because really they had little chance to spot the evil among them and are pretty much powerless to do anything about it.