Vader would have plunged the galaxy into unending civil war, with a constant disregard to individual planetary sovereigns. Rebellions would rise to fight the Empire, but Vader's total lack of guilt for murder would make it impossible for compassionate people to win, especially diplomatically. Vader would be a black hole, enveloping ever star system and galaxy possible. Considering his origins, he needs a continuing driving force to maintain his evil. Without it, I think he would fall into a deep depression and be usurped by those working under him.