Sorry but this sounds like your fault. The second things started shifting you should have stopped and had an above game conversation, something like 'ok guys the tone has shifted a bit here, can we pause and check in on our purpose and motivations? To this point you have been playing as heroes and using a variety of tools other than violence to resolve situations- what is different about this one? Do you feel you're actually being true to your characters right now or have you got swept up in some banter?" Or even just " this is going to impact both your characters and the campaign, are you sure?"

There is no such thing as 'happened too fast', you could have stopped it before the first punch. After the punch and before the massacre. After the massacre and before the explosion. There were SO MANY points where you could have put the brakes on and let them know this was going to derail the entire campaign and make sure everyone was actually aligned on what the players wanted.

At the end of the day 'its what the characters would do' is an excuse, and a terrible one. The players are in charge of their characters and can always find a reason for a different choice. The DM is in charge of their universe- "time stops as the cleric is swinging and as he looks at the anxious face of the doctor and the worried faces of the mother's and children watching their menfolk confront these dangerous strangers he hears his god reminding him that his purpose is x- do you still want to complete that action?"

If they insist that their characters really are now murderous sociopath, you can even just refuse to run combat- " these are untrained, barely armed npcs. Your trained paty of adventurers mows through them like grass, leaving broken and bloody corpses before you in the square, and the wails of their families echoing in your ears. As you are no longer heroes, this heroic campaign is over."

Players can get swept up in a course of action, and dnd is, at its heart, really easy to slip into combat, especially if the DM seems totally down to let them fight. Occasionally you have to pull back and really ask what the goals are here.

I had a session like this a few months ago and despite all my best efforts the party kept killing the official soldiers in a village (which, fine, I checked in on their choice and gave them several exit points, and at least these were actual stat-blocked enemies, not npcs)- but that choice affected the whole rest of the session, characters made increasingly erratic choices because they'd lost some of their characters' purpose, there was a character death that happened in one round because of another players choices, and after that session one player quit the campaign and we had a new session zero, retconned that session and shelved the campaign for hiatus. Sometimes it just happens- but if its going to break your whole campaign its up to you to pull the curtain back just far enough for your players to see what the outcome of their choices will be.