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The events depicted in this question happened many months ago and that campaign is now over. While there is no fixing what I did, I want to move forward and learn from what I am pretty sure is a mistake.
The party I was DMing was full of what I call murder hobos. All they wanted to do was kill everyone that didn't help them and kill every single enemy that ever crossed their path. This made trying to give them information they needed for quests difficult, because anyone who had information ALWAYS died. I actually mean this, they killed every enemy they ever fought without interrogating them.
After a while, I grew tired of this murder hobo gameplay style and I took the group aside and told them that some of enemies have information and so do the towns people, maybe they should talk to people or consider taking someone prisoner to try and get information.
One day, the party attacked some commoners. They killed 9 of them and left one with 6 health1. The paladin2 attacked the commoner and did 20 damage. Because they said non-lethal damage, they thought the NPC should not have died. My ruling was if you do double the health in damage they instantly die (just like a player character) even if you declare non-lethal damage (Massive Damage rule in the PHB).
My hope was that they would realize that they can't just fight everything that comes their way. I will also add that this "attack" wasn't using a sword or any weapon. The paladin trampled this man with his horse. My ruling was that because he did a MASSIVE amount of damage, this guy died instantly. I also argued that there was no possible way he could control the amount of force that his horse trampled that commoner with.
I have been asking myself a lot since this happened, was this a mistake on my part? I feel like I warned them many times that murder hoboing is not the answer and that it will not work, but at the same time that punishment (when I look back) may have been harsh.
Was this punishment too harsh or unfair? If so, what could I do in future to drive home the point that D&D is more than just killing without being unfair to the players.
1 No Experience was ever given when the party killed innocent people. We give out XP on an encounter bases, so at the end of the encounter they get all of the XP
2 The paladin is an Oathbreaker (per the DMG)