Emperor's Demise, Rebel Requirements, and 'Old Ben' vs 'Stormtrooper Han'

Old Ben wanted the Empire to fall because he needed to defeat the Sith. Rebels wanted to fight back against the Empire cuz they were being oppressed. So yea, he did fight Vader, but he didn't fight Vader because he was part of the Rebellion, he was helping out an old friend (Senator Organa) and fulfilling his mission of bringing back the Jedi and defeating the sith. He was too good for some pesky rebellion.