Like the final piece to a puzzle, Sith completes the saga of the rise, fall and redemption of Anakin Skywalker/Darth Vader.
What I find is the most interesting thing about Darth Vader in Sith is how he acts before and after he is places in his black armor.
Before he is in his black armor, DV is strong and confidant that he save Padme, his child and the Republic. He sees his new Sith abilities as a path to do good by overthrowing the Emperor. He fights with all his might against Obi-Won, and to save his own life.
After he dawns his black armor, DV is a broken spirit that only fights when he has too. He is just shaffling through life trying to forget his past. He seems to remain with the Emperor because he feels he has really no where to go or a better purpose to serve. In the end, he is too weak to save himself.
Star Wars uses the character study of Anakin Skywalker/Darth Vader to explain how mankind falls at times and then is redeemed by the next generation. This is the true message of the overall storyline.