He has stated his primary reason was that he was pro-privacy and anti-surveillance. There is no reason to doubt this. He had a huge trove of info he could have sold to China or Russia and he didn't. He has given us no reason to not believe him, and if you're paying attention to Washington you'll see why he didn't go through those channels, especially in the era of shipping off people to be tortured in other countries and "water boarding" is just "coercive means".