I have been thinking lately of the old trope of players vs. players in D&D campaigns, primarily because it has emerged in a few games like an unwanted relative. I have been playing one particular campaign in which everyone is a friend for so long, I haven't had to deal with this.
I've played in, observed and heard about campaigns where this is a common theme. Online comics like to play it up for all its worth, particularly the trope about the thief slipping into the treasure room and pocketing the nicest stuff before the party sees it. You see players killing one another a la Treasure of the Sierra Madre, but its there ... and of course quite a few DMs think its hilarious.
For myself, as a player, I've always found it a little difficult to stomach. After all, I am interacting socially with a person who is either gloating or otherwise behaving like an asshole as they kill something I've worked to keep alive and healthy, and that I've grown attached to ... and I'm expected to simply consider it 'part of the game.' I'm not to consider the action boorish, or proof of a lacking character.
When I was a boy, I had a grandfather whose favorite game in the world was cribbage, and who took great glee in being able to call 'muggins' on my 10-year-old self. For those of you who may not know, in the game of cribbage, if you count your cards for less points than they're worth, and then peg the total you've counted incorrectly, your opponent can declare "muggins" and count those points themselves. Thus, if you are a ten-year-old boy, and you have not played cribbage for sixty years, and have therefore not had all this time to train yourself to be a self-serving asshole, you can expect to lose a couple of points a game to a grandfather whose belief system includes taking advantage of this difference.
My grandfather used to tell me that it would "smarten me up" to the ways of the world, clearly with the expectation that someday I would be able to skin players with less skill - or commitment to winning, I suppose - than I would eventually have under my grandfather's tutelage. It was supposed to teach me to count cards more fastidiously. What it taught me was not to play cards with my grandfather.
If today I found myself playing cribbage with someone who called 'muggins' on me, I believe my first inclination would be to punch them in the face. I wouldn't do that, of course. I haven't punched anyone in the face since grade school. But my second inclination would be to tell them what assholes they were, rise from the table, and never play cribbage with them again. Or anything else, for that matter. I don't associate with people like this.
My parents, incidentally, played cribbage all the time, and the house rule was that if a hand was counted improperly, other people helped to make sure the count was right and the game continued in a friendly, competitive manner. So I guess my grandfather's influence on his children didn't go very far.
It isn't that there aren't people playing cut-throat D&D. It's only that I would like these people to be hit by a bus or something ... if they can't learn the error of their ways, that is, and smarten up.
To discourage this behavior, I usually try to play a wide-open game ... that is, I don't often share secret information with one part of the party and intentionally keep others in the dark. I used to do this all the time - it seemed like fun, taking people out into the back to describe something only they could see, or encouraging players not to be open about their characters, or encouraging secret messages to be sent around the table. In the long run, however, all this really did was to create a level of distrust which eventually disrupted the game and drove off players.
Some gentle readers might react to this assertion with the sentiment that players who would quit a game over something like this weren't good enough to play. Or 'mature' enough. Or some other belief my grandfather would have appreciated, that 'strong' players don't crumble because their characters are killed. That is, that 'muggins' is part of the game, and if you can't play with muggins then you don't deserve to play.
I have found, generally, that without all the running around behind other people's backs, players form a camaraderie that is much, much stronger than backstabbing and player-from-player theft will allow. Games tend to improve with time, rather than fall apart. Players respect one another. Players treat each other with compassion and concern. This doesn't seem like a bad thing to me. It sounds a game that 'adults' would play. Adults who can work for long-term goals and not short-term gain.
And I'm funny. I prefer to play with adults.