Axtually, Axel, it was in a screenshot from KH2. But that's not important.
I used to believe, before CoM that DiZ was the Ansem from the original game, but I have sense revised my theory.
In Ansem Report 12, I think, he stats that heart and soul are separate. If that is the case, the one from the game is simply a hear corrupt. Like Kairi, DiZ is "but a shell." Yet, he is different. He keeps some sense of self even though he shouldn't.
I think this has to do with the poem read in the factory at the End of the World, but as that is cryptic, vague, and probably means nothing at all, I can't say for sure.
Also, back on subject of DiZ being Ansem, Vexen told Riku, "He is Ansem, but not the [Ansem] we know." The one that walks in twilight... which makes me think. Is there heart, soul, and something else?