the devil is not so black as he is painted
but oh, he is painted a dark enough shade
It was fear and manipulation that lead Kaname to Aizen’s service. Sōsuke had power beyond measure, enough ability to erase Kaname Tousen without a trace and get away with it fully. It was only Kaname himself who could pierce through the illusions of Kyōka Suigetsu. Is it not natural, then, that he followed in fear of his life?
But slowly, eventually... It changed.
Perhaps it was getting to know and understand him better, or as well as it is possible to understand a man like reflecting water, but eventually Kaname began to respect him rather than his power.
He started to truly believe in Aizen’s way, perhaps mostly due to believing his victory a forgone conclusion (and it almost was). With Aizen’s victory, he could change the very Soul Society he despised for the better. With that victory, justice could truly be put in place, no longer corrupted by cruel power and nobility.
But most importantly... He was following Aizen out of willing loyalty, rather than force. Perhaps it could be blamed on Stockholm, but it is more than likely he had an insight into something beyond Aizen’s collected and unfathomable image, due to the very same reasons Kaname had been a threat to Kyōka Suigetsu’s power- His piercing insight.
He considers Aizen a comrade and a peer, though not quite an equal. He is aware of how Aizen is very much above him, but knows the other enough that he freely speaks against him should he disapprove, a far difference from their relationship at the start. He sees Aizen, what he is, and... Accepts it. He is not a man, not a god, but perhaps easier to think of as a simple aspect of nature; the sun rises without fail, the storm rages as it does, and Aizen Sōsuke is the power that changes everything, with or without you.
However, he does not think of him as a friend. Oh, never a friend, only a man to follow to victory. For all he knows of Aizen’s nature, he leaves it be, and has simply no interest in any companionship.
Aizen Sōsuke is the king, and he the vassal who advises. And that is how it should be.
the devil can cite scripture for his purpose
but better the devil you know than the devil you don’t