Apparently I have an old account here ^.
The full myth really makes the answer a bit more clear. Prometheus stole fire for man only because Zeus took it from them as a punishment for a joke Prometheus played. Prometheus had the mortals swap Zeus' offering to trick him into accepting a sack of bones and skin wrapped in what looked like good meat. He cheated Zeus out of an offering that he felt was rightfully his (being the King of the Gods).
The offering probably should have been Prometheus', though, as Prometheus made man and Zeus didn't have much to do with man except hucking lightning occasionally and being manager for the Gods who were more hands-on.
So Prometheus did it to atone for a prank that got his creation punished, making him a hero in my eyes because he took responsibility for his mistake.