Thufrat wasn't entirely sure as to why Ark just ran away, but he could make an educated guess, and after seeing the scene when he arrived, he knew that his guess was correct. Thufrat knew that Ark was once a human, and that he still helped humans if he could. Maybe this torture had changed him in some way that Thufrat had no way of knowing, but his nature hadn't changed yet, and if he could help people that deserved it, he definitely would.
With that said, Thufrat knew that he had changed, he could feel it. Ark was just a little bit different, but it was easy enough to notice. But since he knew the entire story, there was nothing for him to ask about, he knew exactly why Ark had changed, and he was even surprised that he didn't change more than what he did. Thufrat was not sure who these people were, but now that Ark had already helped them, they could just leave them alone. They had no obligation to help these people any more.
"Let's go!"
