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.