Xehanort was a visionary. He, who had no past, lived in and for the future alone. It was his gift to see the consequences of an action in advance. He could imagine the world as he wished it to be, and then figure out what steps to take in order to bring that future about.
He naturally became the leader of his fellow apprentices, for they could not see as he could, and what he desired they too desired: knowledge. But he made that desire into a plan.
In his vision of the future as it should be, he was the master of all, the greatest sage that had ever lived. And he knew, with a certainty he could bend the universe around, that it would be so.
What had Ansem ever done to deserve being called "the Wise"? Xehanort, who deserved it far more, took that name and all others from him. It was only fitting: the prophet of the future gave himself a past.
Xemnas could still rule by word alone, seducing or punishing as he willed with words and the magic of his voice and vision, the magic that had made him great before. None of those who served him (for in his mind even the former apprentices were subordinate to him as they had always been, though they had not known it before) would dare defy him to his face, for as he could promise them all that they desired, so could he withhold it.
Never did his conviction waver. He knew until the day he faded that he was the greatest sage that had ever lived or would ever live, and that all the worlds were by right his dominion. The death cries of his colleagues made no dent in his certainty.
He had never noticed when the chill abyss of madness had opened beneath his feet as before those of so many visionaries before him.