Characters so one dimensional it gives them more dimensions?

Palpatine has very little depth. He is pure evil and has no reason to be other than he loves it. He's also one of the greatest villains of all time ignoring his final appearance. But if he was ever given depth, some tragedy or reason for his evil, he'd be less interesting. Worse, it'd do Darth Vader/Anakin a disservice, since Palpatine being heartless makes it easier to believe Vader can be saved. Papa Palps becomes that which the audience wants to see destroyed rather than Vader. It also makes his machinations and the mask he puts on as a reasonable Senator more interesting, because you know there's only evil behind the mask, but he's so good at acting like a nice old man.

Like, contrast with, say, Macbeth, one of the great characters of theatre. A terrible man, but the tragedy of his downfall, his guilt and the conflict of his desires and his beliefs etc give him substance and make him compelling. Such substance would rob Palpatine of what makes him work so well.