There could have been propaganda value in the "lone loyal Jedi", who died saving Palpatine from the evil Mace Windu and the other traitors? That would make Anakin's legacy of dubious value to the Rebellion.
Maybe Vader develops a sort of split personality to cope with the guilt of the terrible things he's done? In the Star Wars novel he actually ruminates on the destruction of Alderaan, almost as if he regrets it, before reminding himself it was a planet of traitors too.
Lucas is supposed to have talked to psychologists in developing the OT, so that may have figured into Vader's mindset and motivations.