Thought and expected, not just from the OT but all Lucas statements, Vader is meant to be a tragic hero, its classic archetype. Tragic Hero's have to be hero's first before they fall to darkness and who are then redeemed. Otherwise they are not hero's at all but merely villains who have a last second change of heart and the audience do not care when they turn good. I do agree with you partially about the story. I felt (and still do) that from a strictly story telling potential the PT could have been greater than the OT. It wasn't but there are reasons for that, which if you listen to the Directors Commentary on the PT films will be obvious.