Silver Surfer was the herald for Galactus.. the "consumer of worlds" - Surfer was the one who scouted the galaxy for worlds that contained enough energy for Galactus..
So, yes in a sense, he was a "bad guy" until he met the Fantastic Four who helped Silver Surfer become a "good Guy" and turn and rebel against galactus.
Not sure how the movie will portray him.. from the rumors I've heard, Galactus is much less of a character and Silver Surfer may remain a bad guy in the movie.