Moral ambiguity did it.
Problems started with the regular crew becoming a revolving door of players, then irregular players, and finally all but one of the original players was gone, and the original premise was generally unknown (while gameplay remained awesome), and consequences, therefore, seemed disconnected or irrelevant. These are all workable.
We had a circumstance in which an encroaching army pressed forward with untenable force, and the team was forced into a position of teamwork or die. One player, taking an opportunity to throw an infinity of monkey-wrenches into the works, threw an infinity of monkey-wrenches into the works.
- Having obtained what he needed to activate the device (which, ostensibly, would have saved the entire gang), positioned his teleporting amulet squarely into the control panel and disappeared along with the device, leaving his half-dead mates to a brisk and merciless doom.
There was a palpable air of sadness when these new players found their carefully-crafted characters obliterated, and a need for vengeance became immediate. Bad feelings permeated the room, barely held back with the delicious white chocolate cookies the shop owner's dad had made. Two players looked across the table and wondered at the actions of their alleged friend, because the fact is that it is very difficult to distinguish between one's character in a game and one's character in life. Yes, there was some acting involved, but I can assure you that neither of the betrayed gentlemen will be lending the fine traitorous actor any money in the foreseeable future.
I have an arrangement with one of my friends, where, when we play a good game of RISK, we have a gentleman's agreement to never attack the other in their native lands, which utterly infuriates all other players; but why? We have tapped into a principle of strength which is this: integrity. All comers quickly learn that they must overwhelm us and form their own alliances to hope to defeat us, even as we wage cold war across the continents. United we stand. That relationship has traversed the game table and proceeded to form a trust between us that is peculiarly firm.
My message is this: please consider that you are not merely playing in a game. You are also playing with the man across from you, and repercussions will attend your actions. You will be known.