And I'll add my two cents: (Major Spoilers)
Organization XIII was created by Ansem, the first six members actually studied with Ansem on the Darkness of the hearts. Zexion was the one who first ever insisted Ansem to begin experimenting with this.
Of course, they all ended up losing their hearts. And later, the remaining members joined. Of course, they were not so loyal. Now Organization XIII seeks to discover Kingdom Hearts and regain their hearts, becoming complete beings once more.
As of now, they all seem bad. Sora battles many, and it appears they want to kill him. So they're probably evil.
And Diz is the true Ansem, the real Ansem. The Ansem we fought in Kingdom Hearts was, in fact, Xehanort, Member I of the Organization. When his heart left his body, he returned believing himself to be the true Ansem, this is who we faced in the original game.
Xenmas is Xehanort's shell. Shell's, obviously, do not always retain memories of their life, and in Xenmas' case, he believed himself to be the real Ansem as well. He discovered the Ansem Reports we found in Kingdom Hearts and thought they spoke of him.
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And this is where my theory comes in: Ansem (Diz) created Xehanort, a living test subject with a shell and false heart. A heart born entirely out of Darkness. This, of course, all happening before Kingdom Hearts. Xehanort, with the letters changed, means 'No Heart', and 'X' in the Organization means 'Betrayal' or 'Unknown'.
Which is what Xehanort is; an Unknown. He does not exist. He, technically, has no heart. For his heart was created from Darkness. It's not real. So, when he tests himself, and casts himself off to Darkness, he loses the memories of before and his heart returns, believing to be the true Ansem.
Which is when his shell is created, also believing himself to be the true Ansem, which is where Xemnas originates from; 'Ansem' is Xenmas mixed up.
So Ansem (Diz) realizes that this was all "A Creation Born Of Ignorance". In his own quest to understand the heart, and the darkness that lies within it. Ruined his life, and his Kingdom.
For, you see, Ansem studied this Darkness as a way to protect his people. Which, as Zexion does, maniuplated his King to run these tests. Zexion couldn't run experiments in Hollow Bastion and not be discovered for it. Running tests on humans would be considered a wrong--doing in Ansem's eyes.
But, if Zexion could persuade Ansem to do it himself, then it would be approved. So Ansem took his six loyal followers, including the newly created Xehanort, and delved deeper into Darkness.
When Ansem realized what he had done, after Mickey's arrival and speaking to him. He erased all his data, destroyed his Reports, which were now spread across the worlds, and went into hiding. Calling himself Diz.
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Of course, that's all just my theory. Hopefully I'll turn out to be right. It is pretty possible when you think about it.
