They get arrested, and for community service, they have to do the quest or get hanged…hung.
Then there needs to be consequences for that. Now they’re in jail and the quest is to escape before the party is executed.
My DM always says, you can do anything you want, but there will be consequences.
The killing was at the end of the oneshot, so i simply finished by saying that they got captured by the authorities and executed for murder.
I had something like that happen. I had to rewrite the whole story on the fly and kept hinting at the big, important mystery that they missed out because they killed the people necessary for the initial hook of the quest.
Did they at least one-shot the quest giver?
I was a player in a campaign where two of the other players just full-on sprinted past every sign post the (Pathfinder) GM threw up saying, “This character is here to give quests. He is much higher level than you. Don’t attack him. He will slaughter you.”
They pulled out their weapons and made to attack the guy and my CN character was like, “You have fun with that. I don’t want to die today.” The quest giver and our other party member tried to talk them out of it for a couple rounds AFTER they started attacking (and doing no damage, naturally). The NPC then started using non-lethal spells to try to defuse the situation. A couple rounds of that and the GM finally gave up went hostile. Two or three AoE attacks later and both characters were gone.
They rolled new characters, but I didn’t last much longer in that campaign.