but there is Zexy's line to Axel: "...We were going to set him off against the traitors, but with Marluxia gone, there is no need. He is nothing but a nusiance now."
if zexion and vexen were planning to overthrow the organization using riku, why would zexion try to dispose of riku himself?
so they only used riku to counter marluxias plan :D
