William wanted to know the basics, so he'd not end up getting screwed at the end by the Cardinals. He was, after all, someone who knew nothing about their world. And if they wanted to mess with him, they'd provide only the useless basic info.
So if William managed to get his hands over scrolls speaking about the basics, he'd be able to discern the value of information exchanged with the Cardinals. As for Leo, he could tell from the way Arthur dealt with Jeremy that the Cult higher ups didn't care that much about those lower than them.
He got that the way the Cardinals asked for him to be brought over to their main base was done to scare him off. Perhaps they wanted to get the identity of the enemy without the need to expose too much of their world. And William wouldn't settle with such a thing.
