Well, presumably it was Vader. He was the hero of his own story. I like to look at villains that way. I think it makes them more dynamic to see them as, at least, believing that they are the good guy, and the protagonists are the evil ones.
He was the dark lord of the sith. Murderer of millions. He single-handedly destroyed the galaxy's peacekeeping legion, and brought chaos to every planet within his reach.
And he thought he was the good guy?
I disagree fully, and the greatest villains of all time had no illusions that they weren't anything
but evil.
Hans Gruber knew he was a villain. Jason Vorheez and Freddy Krueger never once believed they were the good guys. Heath Ledger's Joker may have thought himself his own hero, but he knew very specifically that his anarchy fetish was a pathway to destruction. He was no savior, he was a destroyer, and he knew it.
"I don't murder children."
"I do, when I have to. I'm a monster."
--Malcolm Reynolds and the Operative, Serenity.
Even in this, when the operative believed that everything he did was for the good of the alliance, he had no doubt in his mind that he was a villain. His job was to commit atrocities, in the name of good.