[Mushiro]

If anything, Urahara and Aizen are two men searching for the same thing, but with different intentions. Urahara seeks power to help others and to better himself as a person, while Aizen seeks power for his own self-gain and to rule over others. It's a classic plot line, Good vs. Evil.

Much like Full Metal Alchemist, there were many who sought the power of the Philosophers Stone. Some wanted it for good, others wanted it for evil. In this instance, the Hougyuku is the Philosophers Stone. Aizen mis-read Urahara, he's not in any way the man that he thinks he is. Yes, he has tinkered with the dark side, but only to make light out of darkness.