My players accepted a quest to basically be drug mules, delivering illicit goods to a faraway town. While alone with the NPC giving the assignment, they decided to kill him and take the shipment of illicit goods for themselves. The NPC they killed was part of a large tribe of ruthless orcs, and they knew this when they decided to kill him. They had encountered the tribe before, and were on speaking terms with them.
They buried the body, but what they don't know is that the orc had told his friends that he was making a deal and that if anything happened to him, the PCs would be to blame.
I want the players to realize that killing random people without precedent has repercussions. If they see the group of orcs again, the orcs will want their blood.
What I want to avoid: getting out of this with no consequences.
I don't think that any persuasion roll, or deception roll, would be good enough to dissuade a large group of orcs from wanting to tear them limb from limb. But I also don't want the players to feel like they're being railroaded.
I'm not trying to avoid a battle with the orcs. I'd rather that had to battle with the orcs than be able to lie about having killed one of their tribesmen unprovoked.
What I need help with
I am unsure of how to deal with the orcs not being open to reason with the players without it feeling "cheap," in that they can't use deception or persuasion (ability checks) against the orcs to avoid the angry orcs who want their vengeance.
What a good answer will help me do
It seems fair to have the orcs be too angry to reason with next time they see them. How do I use the game to reflect that?
How can I prevent the party from being able to bluff or persuade the orcs without the loss of player agency?