the whole purpose of these movies IS to show parallels between luke and anakin. when watching the movies in order, we will know the steps anakin takes, and how those actions lead him to the dark side. then we see luke, his son, his only shot at redemtion, following those same steps. it ups the ante, so to speak, as to whether luke will follow the same dark path as his father. from vaders perspective, he can see that luke goes through the same things he went through. but luke does not turn, the emperor does not trick him, luke will die trying to turn his father back to the light side. THAT is why anakin turns. you may say its stupid, but in reality, its superb storytelling.