In the Star Wars movie A New Hope (1977), when Luke meets Obi-Wan in the first time, Obi-Wan takes luke to his home. Then Luke and Obi-Wan have the below conversation:
LUKE SKYWALKER - How did my father die?
BEN (Obi-Wan Kenobi) - 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. Vader was seduced by the dark side of the Force.
LUKE SKYWALKER - The Force?
BEN (Obi-Wan Kenobi) - Well, the Force is what gives a Jedi his power. It's an energy field Created by all living things. It Surrounds us and penetrates us. It binds the galaxy together.
As per the above conversation, what is the reason that Obi-Wan lied to Luke by telling him that Darth Vader killed his father although Anakin Skywalker and Darth Vader are the same person?