This is a problem as old as the game: How does a DM get the players to stop just outright slaying all NPCs, but more specifically the "good guys". Assuming that the PCs are at least sort of good, or at least want open access to good/neutral civilization.
This is not a problem in my Hard Fun Old School Unfair Unbalance style games. So here is what happened over the weekend:
Another DM could not make it to his game, so he asked me to cover for him. He gave me his notes, but we had no time to chat. So it's an urban set game, I'm not sure it it's published or homebrew as I only had his notes. Last game the PCs did a task for an NPC, and the game ended at a big party. This game picks up at the party. The players have fun for a bit and then the plot kicks off: the NPC is found murdered...and the PCs get blamed for it. The PCs surrender and get taken to jail. They get informed that they will spend the night in jail as the judge won't be in until morning. The players panic a bit here and try to escape...but fail. As per the plot, later that night a shadowy figure shows up and offers to free the PCs if they do a job for him. The PCs agree to this magically bound quest. While the PCs could have made a quiet escape....they don't. The guards get alerted and alarms are sounded.
And as the city guards attempt to recapture the escaping prisoner PCs, the PCs just go full blown murderhobo on all the city guards. So this is the good city where a lot of the rest of the game is set, going by the notes. And the PCs getting arrested for falsely killing the NPC, that they could have been found innocent for, does not even matter now. The PCs have now just become the worst mass murderers in city history killing many guards and such.
The players never give any of this role playing any thought. They are LOCKED into the idea that ANY combat encounter MUST be a murderhobo slaughter fest to the death. A guard hits them with a net, they must use thier most deadly weapons, spells and abilities to do a ton of damage and slaughter the guard.
After the slaughter fest, the PCs flee the city and go to hide in some caves. And this ends the adventure for the night. Of course, next game brings up the problem: what will the city do about the most vile and evil mass murderers in all of history. Sure you could just ignore it. But most DMs like to have a bit more 'reality based games' where consequences matter.
I sent the game notes to the games DM, and he was a bit shocked the players did the murderfest. There is a chance, he said, he might need me to cover the game next week. So that puts it back to me of what might happen. My reaction would be the super harsh way...killing the characters. And maybe reseting the game with some time travel or something like that.
But this leaves the issue of talking to the players. I'm not really a fan of talking. They think they did nothing wrong by slaughtering so many NPCs, but then still "get" that they had to flee the city as they are now mass murderers. I know from many past "talks" that nothing much will come from such a talk. I'm sure the players will say "anything in the game that gets in my characters way will be slaughtered!!!!!!", as that is exactly what they did.
But....here I am. Asking for maybe another view point? Is there anything new to say on this topic? I guess someone might say that a game must have a session zero where the DM very slowly and carefully tells the players the way good, evil, slaughter and common sense work in the game. Though in this case it's not "my" game. Still the players "get" that it was wrong to slaughter all the guards......but that did NOTHING to stop them.
So, anyone?
There are a couple of things that might have prevented this, as well as a few ways to "fix" it, to varying degrees of desirability.
Now I don't know what game you're playing, but at least in 5e, by RAW as long as you're doing melee damage you can choose to deal non-lethal damage. There are plenty of house-rules that are feasible to strengthen or weaken that rule. That said, if PCs are purposefully killing "good" guards (more on that in a second), they aren't good.
There are a couple of ways to signpost this to your players before they actually go through the action. They're all fourth-wall breaking and I know that some people don't like that, but I recall those older Bethesda games where you could actually kill plot-critical NPCs, but you'd get a big pop-up warning you that you'd messed up the flow of destiny or whatever and you should probably reload a save. "Reloading a save" doesn't happen in TTRPGs which is why I think it's fair for GMs to provide a bit of a "are you sure about that?" warning ahead of campaign-breaking decisions.
This, of course, brings up a number of other issues, primarily about the risks of planning a campaign arc too far out in advance, because you never know when your heroes are going to turn into notorious villains. There are plenty of ways to resolve this moving forward, but they really should all be done OOC in discussion with the players (and in this case, the GM).
1) You memory-hole the session. We'll call this Plan A. This is not as outrageous or as radical of a choice as it may sound. People will definitely have objections to the mere suggestion, but it is a choice and in certain situations it's a good one. I can see a DM handing their campaign off to a guest-DM for a few sessions, seeing everything come crashing and burning down around them, and saying "well that was a fun little jaunt through an AU, but let's get back to the
real campaign." Not ideal, but it's there.
2) Plan B: The players decide to embrace their roles as villains. This, of course, changes the nature of the campaign, but if it's what the players seem to want, you can certainly roll with it. It might not be what the original DM wants, but there's clearly been a disconnect between the DMs and the players on the expectations on the nature of the campaign. That's something that really ought to be ironed out in a Session Zero and if you haven't got something like that set in stone then maybe you shouldn't be handing the keys to your kingdom to somebody else for a spin. I hope you've recognized that you were set up for failure here.
3) The players decide they have to find a way to redeem themselves. This is a classic trope, I'm sure I don't need to get too far into detail about it. It's also a more deeply involved affair, so we'll call this Plan C.
4) The players actions were retroactively "good". I like to call this Plan ACAB. The PCs, still trying to clear their name from the original murder, uncover rot and corruption so deep that it's infested the very city guard itself. They have, it turns out, unwittingly cleared the city of a substantial population of dirty cops, and the more evidence they uncover, the better chance they have of clearing their names for both "crimes", clearing their names, and restoring their standing in the city. As far as retcons go, it's certainly not terrible. And you can still have at least an NPC or two call them out and/or harbor continuing resentment for slaughtering them all
before finding out they were corrupt. But it solves the problem without dramatically altering the course the campaign while also opening up complications in the relationships the party has with several members of the community. I think I like this one best.