That's ironic. His casting as good guy Gordon in BB came as a shock to many because he's played so many bad guys over the years. Dracula, Leon (The Professional), True Romance, Lost in Space, The Contender, Murder in the First, The Firm (not the Cruise movie), Made in Britain, etc. etc. But I guess if BB is what you first saw him in, then that's gonna stick.
That's not bad either. But I've gotten so used to him as Lt. Gordon ("Batman Begins") that I'm not sure I'll be able to picture him as a bad guy anymore.
Nope, it's not the first thing I saw him in, just the most recent. One that sticks out is Air Force One, where he did a very good job as a crazy bad guy, and also The Fifth Element, where he played an even crazier bad guy. But I still think the skin that he wore in Batman Begins suited him best.
Also, I've heard Oldman quoted as saying (when someone made a similar point to him) that he's played just as many good guys as bad guys, but the bad guys were just in higher profile movies.