Well, killing the Emperor and Vader wouldn't free the galaxy anyway. There are still Moffs, Admirals, and warlords aplenty, with thousands of Star Destroyers to sink, burn, or take as prizes. Even if we're discarding the EU here, there's years ahead of the Rebellion to rebuild the Republic; even the Senate itself has been disbanded for four years, with military governors having power at least that long (and even longer, if the deleted scenes from ROTS are to be considered). Do we really think that they would all just give up their power because Vader and the Emperor blew up on a Death Star that was really more the Emperor's bid for personal power than a real military machine at all? I don't.