I wouldn't be exactly surprised if we find out Xykon was trying to subvert or otherwise work around the ritual because he suspected it didn't do what he was told it would do, but I do think this is an important point to remember. Xykon, as a character, is far more likely to Meteor Swarm Redcloak in the face than he is to pull an elaborate double-cross. He's more of a front-cross kind of guy.