Luke: Why didn't you tell me? You told me Vader betrayed and murdered my father.
Obi-Wan: Your father... was seduced by the Dark Side of the Force. He ceased to be Anakin Skywalker and BECAME Darth Vader. When that happened, the good man who was your father was destroyed. So what I told you was TRUE... from a certain point of view.
Luke: A certain point of view?
Obi-Wan: Luke, you're going to find that many of the truths we cling to... depend greatly on our own point of view. Anakin was a good friend.