As soon as Charles knew everything, there was to know about the current situation. He had ordered Xyle and Ashiel to be on their way. It's not about a task that they need to take lightly. It was a matter of their life and death. They knew that, but still, they have agreed
In the very end, they didn't agree because it was what they wanted to do but agreed because they wanted to save the empire from its impending danger
If they enter the said place, then they are already prepared for the necessary consequences. They never treated traitors and spies differently. Their punishment is the same
After extracting every information they had from the ones who had ordered them to do those, death was enough of a punishment for them
No one in that place will spare their lives in the first place. Charles knew of that as Xyle told him everything. Still, when he was about to point out some of the people, Xyle volunteered himself, following what Ashiel said