This is one of the things I have always found lacking in the Sith belief system, that they did actually touch upon with Yuthura Ban in K1. Villains are always the heroes of their own stories. Very rarely do they simply enjoy power for power's sake, there's always some sort of motivation that is noble in their own eyes. Revan, as well, started out trying to stop the Mandalorians, and then he fell. These are the motivations that are STILL lacking in Ep. I and II. While they're attempting to explain Anakin/Vader's fall, they're just not doing a very good job of it.
Sorry, but I am not 9 anymore, and I just don't buy the "bad for bad's sake" that had served so well. While I am not looking for another Magneto, by any stretch, and I don't necessarily think we need Sion to pull up a chair and show us his therapy notes, I do want to know more about why these incredibly "evil" beings turned the way they did.