He's not lying. He says from a certain point of view. The point of view is Anakin's because in Kenobi Anakin, as Darth Vader, says that he killed Anakin.
Tells the truth:; Jedi hubris:; No, that's not possible. The Jedi would sense it. Tells a Lie:; A young Jedi named Darth Vader, who was a pupil of mine until he turned to evil, helped the Empire hunt down and destroy the Jedi knights. He betrayed and murdered your father. Now the Jedi are all but extinct.
