Personally, I believe that Anakin was in denial about the dark side of the force. This is shown when he is on Mustafar and Obi-wan tells him he has turned and Anakin says no, that Obi-wan is dark. Finally, when he is officially Vader, he understands that he is dark, but it is to late to go back. Also, he is sith, and you have to focus on the now. If a chance to kill your master arrives, and you want to, you take it. no buts about it.