He wanted to become a god and enslave all the bishes whoever existed, it's never been significantly proved why he was evil, maybe path to power, or in search for power is most likely the answer. I guess he wasn't happy with the world, the way tje soul king was ruling it and wanted to kill the soul king and have his own rule and kingdom established, where Everthing would go just as pleases.
Ichigo said he only felt loneliness from aizens sword
From the start of the series, he was shown as nice and caring dude which was only to conceal his true dangerous nature, Aizen is the kind of guy who would use anyone to achieve his goal.
His beautiful appearance also reflects his personality, he's pure evil and cold
If his dad had slapped him a countless times when he was young, he'd be a good douche lol, he might've had a bad childhood, I've seen many anime characters who are evil/turn evil because of their lonely and horrible childhood ....who knows
