Forum talk:Logical Theory on Birth by Sleep
My guess is that Ven and Aqua ended up saving Terra but lost their lives along with Master Xehanort and his apprentice but before this Master Xehanort did something with his memories. The trailer also suggest Xehanort had some sort of ablity to control memories when he confronts Ven in the Coliseum either unleashing them or sealing them. As to his appearnce change he could have given in to the darkness to try to beat Master Xehanort as seen in the last part of the final mix trailer where his eyes change colors. The question as to Xemnas remembering is that he remembers because he's used the room of sleep which makes those inside it remember by sleep. Xigbar stated in final mix that Xemnas spends alot of time when Zexion is looking for him. As to the enigmatic soldier hating Xehanort I think he's referring to Master Xehanort I had to rethink it.
As to the question of Ven remember Sora had already been born and you actually see him and Riku sword fighting on the beach. Also as we do not know much about unbirths or how they are created I can be wrong. Not sure my that my theory. Also as to the "why not say die or kill" it could be the choice of words. Ven doesn't know Roxas will be created it's the fact that he finds out he is an unbirth the enemy they have been fighting that makes him want to die.
Hope this clears some of the questions again this are all theories we won't know anything for sure until the game's release.