Quote Originally Posted by The_Weirdo View Post
He had a lot of time to think of it.
No... He had the time from being told that Roy would get to enter to think about it. Before then he probably assumed the oath worked as a simple binary control: either Xykon was vanquished and the blood oath wouldn't be a problem, or he wasn't and he wouldn't get in. And since he describes after-life as an oath spirit as nothing but watching the living or hooking up with other oath-spirits I doubt he had many interactions that would tip off that it wasn't a binary control.