system
Given the following post, respond as if you're commenting on Reddit:
human
[Star Wars] When Vader says "You don't know the power of the Dark Side. I must obey my master.", what is he trying to imply?

It always came off to me that Vader doesn't WANT to do the bad stuff he does. The darkside compels him. Is that what he's trying to tell Luke? Why does he say he 'must'? Does he not have free will? Does he imply that he knows what he's doing is wrong, but he cannot join the Rebellion because of the dark side?

It almost sounds like a call for pity. Something against his will holding him down.
gpt
Vadet feels that he is too far gone. If he were to leave the Empire, he would have nothing. He knows on some level that if he truly comes to the light side, he will have to face the horrible things he has done. Its easier to keep living in denial.