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Translations: Gintama 507 (2)
By then, he was probably already hiding his true power and had lots of ambition to rule the world, etc. etc. Aizen probably already had a tremendous faith in himself and he knew he had a killer Zanpakutou ability with which he could make his ambitions a reality.
However, as Aizen went higher in the ranks of fifth squad, he also met a guy that could see (no pun intended) through him - Kaname Tousen. (sidenote: Tousen was probably in fifth squad at one point in time. He was most likely referring to Aizen. http://www.onemanga.com/Bleach/176/04/ )
My guess is that, whatever schemes Aizen was pulling at the time, Tousen discovered them, because he was not affected by Kyouka Suigetsu. I cannot imagine that Tousen, with such a fixation with justice, would have let Aizen off the hook. I think he either tried to stop Aizen himself or was about to report him to the Gotei 13. But Aizen was somehow able to convince him with his own reasoning that his way, the order that he would bring to the worlds of Soul Society and Hueco Mundo, was true justice. That the Gotei 13 and Soul Society as a whole are flawed and a new system needs to be put in place to prevent further bloodshed. Aizen's a smart one. I think that, even without Kyouka Suigetsu, he would have been able to make Tousen believe in the sincerity behind his actions. And thus, Tousen became Aizen's accomplice, followed by Gin.
So here's what I believe: Tousen initially didn't have any value to Aizen, but when Tousen discovered what he was up to, Aizen managed to convert him to his side, partially because he didn't want to draw any attention by killing Tousen and partially because he needed a competent accomplice. So Aizen managed to make a servant out of a potential enemy.
So I still thinks in some way he still needs Tousen.......