When Lucas made his first trilogy, I don't believe he had the entire story with two trilogies in mind. But by the time the story was told in its complete grandeur Star Wars turned out to be the story of Darth Vader. From the birth of his Jedi beginnings through his journey of turning evil and then to his retribution.
Because life doesn't fly past us in reels of black and white. There is always the one in the fifty shades of grey.
For me, as a writer, the profile of a villain is very interesting for it is the villain who makes or breaks the story. Darth Vader, (or Anakin before he turned to the dark side) was a brilliant Jedi. His skills were almost unmatched. Superlative so to speak. But what brought about his doom were the things that bothers all of us at some point or the other. Complexes within oneself, death, losing a loved one to things unknown, fear of some kind or the other. Such issues bring all of us down. Turn us to the dark side. However, the prolific nature of Vader's powers was the very same quality that made his turning to the dark side, that much more trenchant.
Darth Vader showed me that talent and achievement has nothing much to do with one's moral strength. It's sometimes unnerving to get lessons from villains like this.
"I find your lack of faith disturbing."