So hear me out, because this is probably one of the craziest things that's ever happened in one of my campaigns. I'll also need to give general backstory, so that there's context.
Quick exposition: My current campaign is set in a steampunk-ish world, and there's a large group that more or less runs half the world, known as the New World Order. They're not the nicest people, with harsh laws and such, but they do their job. Every PC has a grudge against them due to backstories, as well. The N.W.O. are at war with a faction known as the Grayman Empire, which is run by people known as (surprise) Graymen.
People have made independent cities as well, away from these places and their laws, and are able to hold their own against both the N.W.O. and the G.E.. The PCs recently saved a man from a town that was destroyed, and he asked them to take him to his sister, who happens to run one of these independent cities. Upon arrival, they noticed the it was obviously more of a pirate town, as a dead body with a warning to follow the rules was hung at the entrance, and they went shopping. The shopkeep's prices were staggeringly high, since it's a pirate town, and they attempted to intimidate him. A guard attacked them, but they killed him. The shopkeep wasn't very fazed by it, and they demanded free gear from him. They ended up killing him too, as he also attacked them.
After this, there was a discussion about them simply wiping out the ENTIRE town in the name of bandit killing and loot gathering. 2 PCs were all for killing the town, and the other 2 PCs were against it. They want to wipe out the town because, essentially, everyone's a pirate and definitely deserves to die, and so they can be looted. Other PCs argued that it was people taking refuge from the N.W.O. and G.E. that wanted to live away from these laws (which is true, but that doesn't mean they can't defend themselves).
What I tried explaining to them was that this single town, with little outside help, can hold it's own against 2 MAJOR factions of the campaign, employing mechs, ships, and hired guards. They still pushed for the idea of wiping out the town, which comes down to a hell of a lot of work for me, since I would need to map an ENTIRE city for them to kill their way through.
I can set them up with insurmountable odds, but if possible, I would like them to simply surrender / get talked down as soon as they leave the store by either the leader of the town (Morynth), or a huge amount of forces that has congregated. They need to work with Morynth to reach their current goal, and this complete 180 in their attitude baffled me so much we had to end the game early so I could plan.
What I'm asking is: How can I get them to step down from this? Since they saved Morynth's brother, if they surrendered without killing anyone else, they would be able to continue with how the campaign is planned. However, Morynth, since she's the overlord of this town, can't simply appear in the market after there's been a shooting 2 minutes ago and talk them down. Even dropping in a large amount of troops might not dissuade them, and if they decide to kill even one, it wouldn't be fair to the game world that they're let off scot-free. It also wouldn't make sense for a large amount of troops to suddenly be outside the store. Sure, there would be some, but not a whole platoon to stop them. I need something that will stop them, but do so without them killing a dozen people beforehand, as well as keeping it realistic to how fast the town's response would be.
TL;DR: Help me stop people from murdering an entire city of friendly pirates because the shopkeep was a dick with bad prices, and make it clear that if they fight, THEY WILL DIE.
Best Answer
I can see a few ways to play this off from in-world solutions:
The first is have the door get kicked in, soldiers busting in from all sides at the sound of a body hitting the floor, and actually have them surrounded by a lot of guards who, actually cease being hostile very quickly. Instead they just look confused and call out for their commander. The Commander demands to know why they just destroyed his investigation of a potential Gray sympathizer and jeopardized the entire city now that their one thread to plug up a potentially lethal leak has just been pointlessly murdered. Regardless of their answers have him say they'll need to speak with Morynth (I'm assuming they'll know the name) about everything and have them lead away while the shop is ransacked for clues. If they want to steal something at this point, just let them, but also have it all be watered down placebo effect trash since it wasn't a real shop and the prices were high to keep out anyone that shouldn't be there.
The second option is to have them find the evidence that this guy was responsible for partially arming the guard with their weapons and materials. Tell the smartest person who sees it that the amount of steel, leather, and food they were bringing in implies that they look like they're feeding a small country, more than just a large city. Like they're arming every person in town or something insane with dangerous tools, poisons, and god knows what else. Maybe write up a few lines of stuff encoded by cypher with Morynth's name beside it, but with unprotected parts like '300lbs Bat Guano' and '400 crates of bronze wrapped ash-tree sticks'. Just this huge in game hint that this isn't a city, it's a massive pirate barracks that (if they decipher the notes from Morynth) has a failsafe of some sort of newly created construct that should 'be able to protect our city for all time'. Pretty much like what others were saying, and underscore that this is going to get them killed with sheer numbers, just do it in game.
The third option is when they get out for them to see someone in town actually being strung up as a traitor by the city guard. The guy tries to fight back and manages to break free. He runs into an alley directly across from them in plain sight, where they hear a thud and gurgling as a thin homeless man drags the man out of the slight shadow, throat slit and thrashing as he chokes to death on his own blood. Don't even let them roll for his presence or anything. The homeless man greets them with a checkerboard smile and tells them "You'll be wantin to tulk to that man over there to get the punishman for yar murdur. Word travels fast whennin yer shoutin' at eachotha'. Jusa'n remembahr to not be startin' trouble or you'll end up like this pur cutter." Similar to the above but it undercuts them and lets it sort of be known that the power is kept in the city because the homeless, and who knows who else, are the secret eyes and blades of Morynth, and their little scheme isn't exactly a secret and the town's ready for them. Now are they really ready for a town where people are this stealthily murderous? (This could backfire of course, but I bring it up as a combination of a few approaches because it also implies there's a problem with spies and you can use that later with Morynth.)
There are more options of course: Man's not really dead but undead — if you like the cliché. That the place they're at is actually cursed so that killing someone unjustly causes them to become diseased and they need Morynth to tell them the cure.
Really, for a small unorganized pirate place to hold off two major factions is going to need either major firepower, or a hack of some sort — usually magic. And more important than those — a reliable information network. Any and all of those will shut down the group, now it's just showing it to them. Heck, have the welcome brochures that they guy was optionally selling (3 plat for 'The Big Helpful Pamphlet to not get killed') tell them the most minor of this stuff. Heck, have it point out the armies that have lost to this city.
Some of that works better for 5e or more limited games, but the concepts still apply. If this place is there it's formidable, and unless they're that arrogant they shouldn't start anything. If they still try and ignore these hints then the sit down should definitely happen. If they say they want to do this and ignore the sit down? Let them all roll a Wisdom or Dex check/save. Those that beat a 20 manage to escape the ensuing violence alive through forethought or hauling tail sooner than the others. Those that fail are dead. It's a bit brutal, but by that point if the loss is certain, the roll's just politeness.