Quote Originally Posted by Pretentious Soul
Yea yea I know all that, I was just saying those were some really good theories. Yes I'm totally sure that Zexion was killed until proven otherwise but come on you gotta admit!!! The theory of him fighting back when the screen fades to black isn't half bad. You weren't feeling it? Don't be hatin on Zexion
No, those weren't really good theories, as they don't hold up against common sense and what the game tells us.

Zexion is dead. Not only is there the Jiminy's Journal proof, but also if you look at the Proof of Existence area in KHII you can see that Zexy's symbol is as faded and destroyed as those for every other member of the Organization who has been killed, as opposed to the symbols of the members who still exist, like Roxas.

And I'm sorry, but he didn't fight back. Not only is there the info that SeeDRankLou just gave, but you should also remember that he had just nearly gotten killed by the real Riku. He wouldn't have stood a chance against the Riku Replica.

Quote Originally Posted by Pretentious Soul
Neither is the theory of him killing off the riku clone and impersonating him throughout the rest of Sora's story.
Oh, BTW, at the point where Zexion is killed by the Riku Replica, Sora's story is already over. The scene where you find this out is the scene where Riku and Zexion first meet.
Riku: "One of the scents has died. A really strong one!"
Zexion: "That was Marluxia, keeper of this castle. Or ex-keeper, I should say. The Keyblade master took care of him."
So the idea that Zexion imitated the Riku replica for the remainder of Sora's story was clearly put forth by someone who didn't bother to watch the cutscenes anyway, but just went "OMG! Zexion's so awesome!"