If this happens in a Competitive-Collective campaign, there's still been a violation of the social dynamic, because the player introduced a character who didn't fit into the collective and because he purposefully caused the campaign to end unilaterally. That said, since this game is partly Competitive, and there's a strong argument that you ought not intervene with GM fiat. I think the best way to handle this situation is to have a quick player vote - rewind events or let them stand.
If this happens in an Individualist campaign, then it's possible that nothing wrong has happened. The party wasn't required to take in the new player character, and characters aren't required to be nice to each other. As GM, your responsibility is to remind the betrayed players that this is the sort of game they're playing and they agreed not to get angry out of game. However, it's also possible that the traitor has just gone out of his way to purposefully wreck the fun, not because of what his character would do, but to get back at the party that let him die. Resolving this can get complex!
My general rule when addressing the solution to the "TWMCWD" problem is that it doesn't matter whether the problem behavior is "actually" what the character would do! It doesn't matter whether the player is a consummate role-player playing a black-hearted villain, or just a jerk. All that should matter is whether it's a problem.
In a Collective campaign, there's an implicit agreement to create characters that are "one for all, all for one," so if a player makes a character who is a sociopathic traitor, he has created the problem. If he purposefully chooses to role-play a jerk when everyone else has agreed to role-play nice people that means ... he's choosing to be a jerk. And the better he is at role-playing, the more of a jerk he chooses to be. If you're running a Collective campaign, you don't want jerks, whether they are actually jerks or pretending to be jerks.
On the other end of the spectrum, in an Individualist campaign, the players have implicitly agreed that it's "every man for himself," and that they won't let treachery in game make them angry out of game. But if one player behaves in such a way as to make it impossible for the other players to not be angry, then he has created the problem. For instance, imagine that in the last six campaigns, a hypothetical player named Bob has always played a sociopathic thug who always tries to kill the party when they are weak. In the seventh campaign, the party members can hardly be blamed for murdering all of Bob's characters on sight. Bob might be "in character" in his treachery, and the party members acting totally out of character in dispatching them, yet it's Bob who is the problem.
Next Column - In Two Weeks!
Next column we'll continue our exploration of social problems and pitfalls in role-playing games. Note that we've switched to a bi-weekly schedule for Check for Traps, so see you in a fortnight.
Alexander Macris has been playing tabletop games since 1981. In addition to co-authoring the tabletop games Modern Spearhead and Blaze Across the Sands, his work has appeared in Interface, the Cyberpunk 2020 fanzine, and in RPGA AD&D 2nd Edition tournament modules. In addition to running two weekly campaigns, he is publisher of The Escapist and president and CEO of Themis Media. He sleeps on Sundays.