I keep going over and over the lines, “I am not your failure, Obi-Wan. You didn’t kill Anakin Skywalker. I did.” and how they fit into Vader’s mindset, that this is the moment that Obi-Wan truly lets him go, but I don’t think Vader wanted that, he screams Obi-Wan’s name as he leaves, he wants to continue blindly focusing on only Obi-Wan until Sidious has to yank on his leash to get him to stop. And even then he can only hide it, not stop it.
I keep thinking about the moments before those lines, Obi-Wan seeing his face, saying “Anakin.” and “Anakin’s gone. I am what remains.” and the devastation on Obi-Wan’s face to see him this way. I don’t think it’s just that Vader is rejecting Obi-Wan’s pity for him, he isn’t just saying, “Don’t you fucking feel sorry for me, the dark side is more powerful, I have become so much more than I would have been as a Jedi!”
But instead I keep coming back to that Vader cannot let go of Obi-Wan, that he wants Obi-Wan to suffer, he wants Obi-Wan to be consumed by his hate for Vader, not his love. Vader cannot accept love because it would mean that he would have a way back to the light, he cannot do that because he cannot face what he’s done, and he will always know what he did if he accepts Obi-Wan’s love.
But he cannot live without Obi-Wan’s love, either. So he has to try to twist it the only way he knows how, by burning it into hate, like his own love has been burnt into hate. The only way Vader can accept Obi-Wan, the only way Vader can not be in a one-sided relationship with Obi-Wan, is to have Obi-Wan join him in hating each other. “If you loved me, Obi-Wan, you would hate me as I hate you, because I loved you.”
He can’t be Obi-Wan’s failed Padawan, the one Obi-Wan still loves, he has to be Obi-Wan’s nightmare, he has to be the victor over Obi-Wan’s faith in the light and Obi-Wan’s love, he has to be the one to tear down Obi-Wan’s love for him as he feels Obi-Wan tore his love down, on Mustafar when he refused to join Anakin or just walk away. He has to feel Obi-Wan betrayed him (tore down his love for Obi-Wan) on Mustafar, because otherwise Anakin has to face what he’s done.
But all Obi-Wan gives him on that rocky moon is more love.
So Vader tries to break it, tear it apart, say I’m not someone to love, I’m someone to hate. But Obi-Wan can’t, so it breaks the last hold Vader has on him, and Obi-Wan leaves, but that’s not what Vader wanted, he wanted Obi-Wan to join him in this suffering hatred, not leave him.
So he screams Obi-Wan’s name as he leaves, because now he’s alone in the dark again, with this one-sided obsession with someone who he tried to break, but wound up broken by instead.