I forget which book but it was when Vaders grandson was turning dark,they spoke of his grandfather and how as Anakin he was powerful but as Vader because he was more machine than human he could never attain his full potential.As far as bringing balance to the force, it all depends on how you interpret it. He helps to wipe out almost all the jedi but in the end sacrafices himself to kill Palpatine and sve his son. Who intern recreates the Jedi with a more open mind.