Well the "Empire-as-Good" theory is just for fun or playing devil's advocate.

Star Wars as it stands alone is a very black-and-white film (which is proven in the opening scenes of Star Wars by the black contrast of the imposing, inhuman, armoured, obviously-evil Darth Vader against the stark white walls of the Tantive IV and the "soft", feminine, human, obviously-good Princess Leia in her white dress; because of that, you instantly know who's good and who's evil). It's very simple: the good guys are the good guys, and the bad guys are the bad guys. The good guys don't have to show remorse, because they're killing bad guys; it's like in movies where they kill Nazis, you don't have to show remorse, because Nazis are just evil, no other explanation is required.

Also there's really no cause for them to kill Owen and Beru, or to blow up Alderaan over some stolen plans; that's hardly an eye for an eye. Plus the Empire did the whole "Great Jedi Purge" thing which was pretty evil just to be evil, Anakin was murdering little children and stuff; that's not really a "good guy" thing to do.

Grey areas only enter into it when you get into the EU, but the films themselves are pretty cut-and-dried as far as who the good guys and bad guys are.