... Think about it, Superman basically becomes good because his parents loved him and showed him to care for people. Hancock perhaps could have landed, but never found love? maybe he was a foster kid from home to home? What would a person who grew up like this but all of a sudden started getting superpowers be like?
Maybe they would turn criminal? Maybe out of some sense of pity help people once in a while, but not really giving a damn? Maybe he'd become some sort of anti-hero? Punishing those he believes have hurt him?...
Well you clearly wanted to see a different film that what the film-makers wanted to make... It happens all the time.
I personally liked it a lot. Tastes differ. Did we get a different film that what the trailers showed? Yep - and I'm glad for it. I like to be surprised. (The trailers showed no scenes of the last 3rd of the movie, which is sooo nice for a change)
(You could always try write your own story along the lines out outlined... it does sound interesting as well)