I am a little confused to the whole question of how Vader turned to the light....it was very clear back when I saw it as a kid......where is the question?
It always appeared to me that Vader was totally under the Emperors influence. He never once was trying to KILL Luke. He was bringing him to the Emporer, as the Emperor told him to. "You will bring him before me" " "He will come to me??? "
Vader tells Luke they should join and kill the Emperor... Because he knows Luke will kill the Emperor as he is more powerful than him.
If anything , LUKE tries to really kill Vader! Vader is toying with him trying to break him mentally down.
When Vader realizes Luke is not going to fall to the Dark, after he is beaten down so quickly, he knows that Luke can kill the Emperor and this is his chance to escape the Emperors power over him. I also felt the only reason Vader even attempts it is because Palpatine is distracted usimg all his power to subdue Luke.
All of this before the PT even came along.
Its even more clear that Palpy has a force of will over Anakin by telling him what he wants to hear. I always viewed Mace's death as a moment where Palpatine got considerably stronger because Anakin gave in and attacked Mace . This joining up with Palpatine, created a link between them where Palpatine is the main beneficiary of the pair. And his influence over Anakin was enhanced.
Only thing that got through the control was him watching his son be tortured to death.
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