The dragon in question used to be a God and has been helping the party work up to defeating the BBEG (note that they still need him to complete the storyline at this point, but I can find a way around this most likely). They have since become motivated to kill him and I have no idea how to handle this.
Do I let them? Do I punish their stupidity? They are aware of the danger of facing a buffed ancient silver dragon at level 6 so some players have begun looking for alternatives to doing this including bargaining with Asmodeus or sending them to the first level of hell, I can easily get the dragon out of this by relying on the other Gods but I'm not sure whether to. But I know for certain that them trying will either lead to a TPK or his death.
Should I let them do this, or force them to not, and if I do let them do I let them win?