This depends on whether or not you believe that Lucas had the prequels as a part of the story back when he wrote "A New Hope". I personally don't think he did. There are too many plot holes between the original trilogy and the prequels. That being said, and accepting 1-3 as part of the story, which they are, I think he is fueled by anger at himself. He believed he killed his love, and it turned him into a monster. Now, before the prequels were introduced, I believe that he was motivated by his job, and his good side was only brought to the surface by Luke's faith in him. And we all know, Vader finds lack of faith...well...disturbing.