Deadly Computer Blog

Ok, there are four theories here, and only the first one i think seems viable.

Anakin Skywalker/Darth Vader was only evil from the point he helped Palpatine kill Mace Windu (Episode III) to the point where Palpatine told Vader he had living offspring (Episode V). Before this period he was good as a boy and apprentice. After this period he was good and in cahoots with the Rebellion, giving Luke the needed training to become a Jedi Master. The only person who did not realize that Vader was good during this time period was Luke Skywalker. This was necessary because Luke had to believe he was fighting an evil Darth Vader to complete his training.

Actually, it seems very plausible, almost as if it was meant to be.  All the proof is available here. The other theories are forgettable.