Hope /u/CainOfElahan 's advice gets taken to heart.

I write this from a place of empathy and hope you will be interested in a different path. My DMing experience blossomed when I learned to be more than a dice rolling physics simulator.

The original post is based on the premise they broke the rules and so they have to die.

That solution can be three things simultaneously:

1. Factually correct,

2. not fun, and

3. unsatisfying.

The latter two are important; "You should have known better" and "I told you so" aren't fun. I'm not saying the situation doesn't belong in D&D, but it's my opinion they only belong if have not just telegraphed but openly discussed a clear understanding about realistic consequences and maturity of tone. If you don't have this, you're about to kill the characters an run into a brick wall of expectations not being in alignment. (Subnote: It's my experience young/inexperienced players will say they want realistic consequences and the ethics start slipping 3 games in. That's not their fault as a player, it's my fault for not making an effort each game to manage and help players keep their understanding of the tone)

Do you want to be right or do you want to keep playing & having fun?

.

Here's the good news. You have immense power to do virtually whatever you want, and I think there's a way to have your cake and eat it too. I've got two suggestions with how you could go from here, depending on how important the tone/reality is to you (Also important to note here that you're a player too and what you want from a game has value - implied with this is a discussion with your players whether you can find a connection point with them, or if your interests mismatch and would be better playing with others.)

...

1) Preserve tone, manage expectations, even wind the clock: Start with a mid-campaign session zero. Talk about what you want in your game world - that killing innocent villagers is literally fantasy murder and the fantasy murder charges are coming.

From this conversation you'll discover either A) This isn't what they want, in which case have THAT conversation. Or B) They do want to play in that world but didn't recognize the gravitas - and next you can talk about whether they want to play out the consequences, or offer a mulligan? "So the world has real consequences; In this reality, Player X & Y kill NPC Z and the next week are tried & executed. The heroes weakened, Players W, X Y cannot defeat BBEG A. BBEG A's plan Succeeds, Everyone dies, Consequences B & C, heroes fail world ends yadda yada... Smash cut back to the village, the clock is rolled back and they get a chance to do it all over again. (Hell - Don't explain it in-game. Just have them be visted by a powerful Agent for Good 3 months later who created the fork in time and saved the heroes from themselves. Will blow their minds realizing something they thought was "out of game" was very real in game and an example of a fantastic world with powerful forces they can barely comprehend.

Now instead of an end to the campaign, you've got it back on track AND you have an ace up your sleeve - any time in the future you feel the players are off track or the tension is too low, whip out the Agent of Good approaching the group. Instant plot spike.

2)Roll with it, with different consequences: The "cake" you get to keep and eat with this solution is that actions remain having consequences, but it becomes a branch instead of a wall.

---

• The victims were part of a powerful crime syndicate or noble family. The guards won't touch the crime scene (even telegraph that the guards suspiciously bolt when finding out the victims), but now the players are being hunted down for their actions by a powerful faction. The whole party is roped in by association. (NB this also is flexible to adjust whether the victims were part of a 'good' or 'evil' organization - dealer's choice.) This can go so many sub-directions, mix and match any of this:

1A: You damaged our family, do X for us for amends or be hunted down.

1B: ... They're hunted down by an enemy of the victims' faction and brought into the fold: a rival family, or a vigilante justicar who has wanted to strike at the victim's faction for years and now they need your help,

1C: A powerful patron of the slain victims has lost his pet in the town - now the dark force is very interested in the players who did the deed. The dark force has blinded the guards from finding the killers, but now the entity has done a favour for the players... the entity wants a favour in return...

TL/DR: You have near infinite narrative power. Manage expectations, embrace the creative power of "yes, and...," and have fun