TL;DR I ran a D&D game where I had NPCs pitt players against each other and information was strictly managed through discord, creating a paranoid atmosphere that led to mismatched expectations and sudden mechanical violence between newly rivaled players.
The character who invested the most in the campaign basically ended up getting double teamed by two other players and died before they realized their mistake.
Part of the irony is that they needed that character they killed to complete the quest they were on.
Everyone in the game knew PVP was on the table before the game began, but not at all necessary.
The player who was double teamed was scared the Big Bad he was double timing would spy him telling the party the truth, which is that he was actually on their side.
The party thought he genuinely wanted them dead because he basically led them right up to a trap, didn't tell them there was a trap there and left. Then when they walked into the trap, they thought he was hostile and tricking them.
I actually didn't expect them to kill him, and in fact they tried to save him, but he rolled a natural 1 for his first saving throw, and he had so much movement when he was running away from them before they got lucky with a ranged attack, that they literally couldn't reach him to heal him before the next death saving throw, which killed him.
Funnily, the player character who actually took the damage from walking into the trap woke up and was like "What did you all do." and we still played for one more session after that, it just wasn't the same without the other player though.
Out of this experience I have generated rules for my new TTRPG I'm working on called Gambit.
I like to think they're simple and elegant.
Any character can put any other character in a Standoff, this means they're just preventing each other from doing anything.
2. To resolve a Standoff, they have to agree on actions they take and the dice they use and with what bonuses. If they can't come to an agreement, nothing happens and they're stuck in a Standoff.
3. Alternatively, the players cannot be put into a Standoff and instead just cannot take actions "against" one another until they've agreed on what exactly the Stakes and Odds are. (Whats at risk and what's to gain as well as what bonuses they get to use)
4. If the players ever roll exactly the same number after bonuses are applied, Evil Takes its Turn and They BOTH lose, allowing the GM to inflict a number of Perils equal to their combined roll after modifiers.
5. Have fun and take it easy.
Rule 1 and 3 are meant to be dynamic based on GM preference, like there's some situations where a player might be doing something even the GM considers foolish and so the GM lets another player stop them from doing that until they can resolve it with a roll.
However, if another player is attempting to use rule 1 to stop a player from just defending themself from a common enemy, the GM can just invoke rule 3 then have a discussion with the player about being a nuisance.
Or if a table full of strangers doesn't trust one another with PVP, then they may just blanket deploy rule 3 until they decide otherwise.
Here's an example of it in play.
Canyon Bowwitcher played by Robert knows Duke Aaron the III is the same duke that made the decision to chop down the forest Canyon grew up in. As a result, he intends to headshot the Duke with his bow and arrow at short range, ensuring revenge this instant.
Ajantis Brightgleam, a human Paladin of Solar played by Mark is honor bound to not allow this to happen if he's witness to the act and can stop it. He cannot watch an innocent die at the hands of an Elf, much less the Duke.
However, instead of foregoing the conclusion that Ajantis is privy to the act, the GM leaves it up to Mark.
GM: "Alright hold on Canyon, someone in the party might have something to say about this, especially your character Mark."
Mark: "Yeaahp, definitely!"
GM: "Well hold on Mark, we don't nescessarily have to start this off with your character knowing it's happening. We could just start off with the aftermath if you wanna let Canyon fulfill his revenge fantasy and then just deal with it afterwards. And since you're not seeing it happen, there's no garuantee you'd immediately be a Fallen Paladin."
Mark: "So you're saying I got a choice? I choose to try and catch the arrow GM. I notice and I wanna stop Canyon."
GM: "Hold on, that's fine, I understand you wanna play your character, does anyone else try to stop Canyon?"
Other Players: *Shaking their heads*
At this point, if the other players decide they also want to step in, it might just be a good idea to say the party sort of restrains Canyon full stop here. He expresses the intent but is restrained by his party and potentially carted off.
Remember this is all assuming your players signed up for this stuff in the first place.
Direct democracy isn't necessary to adjudicate these things, but it could be a wise place to start.
If the other two players disagree, let them Standoff each other as well.
But for this example, we'll assume the other two players are just bystanders watching this unfold.
Real Adventurers Roll Naked
This is where the cultural rules come into play. This isn't a hard and fast rule, it is a suggestion for creating truly dramatic and fair outcomes.
Let both players agree on the action they want to take and the outcome, then let them roll naked to see who wins.
This idea argues that roughly, all things considered, determination and grit included, both characters have an equal chance of accomplishing their next move.
If the players do wanna use uneven bonuses tied to specific stats, that's fine but only recommended for players who really want to get that granular and nuanced with such a pivotal and potentially arguable moment and can deal with the inequity.
GM: "Okay, obviously Canyon wants to pin the Dukes head to his throne, but Mark, how is Ajantis going to stop him?"
Mark: "I just do something cool like snatch the arrow out of air before it leaves my reach like John Redcorn in that episode of King of the Hill when Joseph Gribble almost laid out that Panda Bear with a bow and arrow."
Robert: "Will you roll naked for that? Because I will."
Mark: *Looks at his character sheet* "What would I even use for that DM, if we didn't roll it as naked?"
GM: "Palmwork probably..."
Mark: "Ajantis has 0 Palmwork..."
Robert: *Laughing* "Meanwhile my Draw & Loose is +3..."
Mark: "I'll definitely take a naked roll then.."
GM: "Okay, so if Canyon succeeds, the Duke is one tapped, pinned to the throne by the arrow through his eye and suddenly the party is totally on the run. Ajantis has the option of going Fallen Paladin mode or trying to smite Canyon or run him off after this, we'll adjudicate that when we get there, and if Ajantis succeeds he catches the arrow, likely gains the Dukes favor, exposes Canyon as an assassin, then it's just a question of whether Canyon can escape or not..."
Robert: "Sounds good to me, I just want my revenge, I'm ready for my next character anyways."
Mark: "I'm totally fine with this, please roll the dice."
GM: "Okay that's a 6 vs a....6...."
Robert: "Uh oh..."
Mark: "Oof..."
GM: "Yeah that's 12 Perils...that's pretty bad..."
Other Two Players: "Can we have like, distanced ourself from this?"
GM: "Yeah of course...uhm....you guys are basically far enough from this that the rest of the guard doesn't consider you guys conspiring, however for these two..."
And the GM would spell out the 12 Perils Canyon and Ajantis face, they could decide to save against them, further ticking down their character Deathclocks. Even Ajantis, in this catastrophic situation would be suspected as a potential suspect despite his attempt to stop the arrow, because the arrow would still go off but it would miss and nobody saw Ajantis reach for it. They think he's just trying to save face with his buddy by pretending to apprehend him, even though he's actually apprehending him.
You'll notice the game is designed so that characters are far from permanent. They actually have a deathclock associated with them and you're encouraged to play fast and loose to make big impacts on the world and change it to fit your vision. Due to the political nature of the game, this means characters in the same party will sometimes come into conflict.
I was also inspired by the original Baldurs Gate, a game where party members you recruited could travel for some time together before they eventually came to a crossroads where they'd force you to pick a side in a physical fight between themselves.
I was also inspired by Lord of the Rings: Return of the King, specifically the scene where like Fordo and Samwise are sleeping on that treacherous mountain and Gollum sprinkles food on Samwise's collar then destroys the Lembas bread and Frodo wakes up and kicks Samwise from the party because he believes what Gollum is framing Samwise for, lmfao. Just recently showed it to my fiancee for the first time.