Death

After his plan to use Riku backfires, Zexion escapes to a different room badly injured. Axel comes in with the Riku replica Vexen made. Naturally, the replica startled Zexion since they look the same and zexion had just been attacked by the real Riku. Axel then tells the replica that he could become his own person if he attainted a power that the real Riku didn't have. Then, Axel holds his hand out at Zexion, pretty much offering him to the replica. He said that Zexion would be a good place to start. The replica moves to attack Zexion and right before he does, the screen turns black and you hear a sword slash.

In RE: COM, his ending is a bit different. We actually see Zexion getting the life drained out of him by the replica. Though we still dont acutally see him 'die'. But it is hinted more that he does.

R I P

With the new info released about kingdom hearts ii: final mix, i now believe (hope) Zexion may still be alive. Its not very likely, but you do get to fight him. <3