I’m about to talk some on Episode 3 of the Star Wars Kenobi series, so move along, there’s nothing to see here.
The shadow-story we are apparently given is that our boy Anakin goes from hate level 9000 to oh crap I really botched the last twenty years of my life what do I do. That’s a pretty good story on its own. Vader in Rogue One is absolutely terrifying, wrecking everybody’s day on that ship. And he’s still at it on Mapuzo. He radiates an aura of Witch-King fear. Just being in the vicinity means you’re likely coming away maimed or dead.
Clearly he manipulates the bajoolies out of the Inquisitors too. There’s a whole pyramid of scary tyranny there. Okay, you say, that’s how the Sith do business. Quite right. And yet…
Vader slowly, meaningfully torments Obi-Wan. Have you ever experienced, or been in the orbit of, chronic pain? I have, and the way people respond to it isn’t what you expect. Let’s rewind the show a few scenes: Anakin is in the bacta tank, a ragged piece of flesh hanging on threads. You know what that says to me? It says he’s in massive physical pain all the time. And look in. The shell is a mirror, his external condition a sign of his greater internal anguish.
There’s a sympathetic, connective force (ahem) at work here. Anakin wants Obi-Wan back. It’s not so much that he wants Ben to suffer. I mean, he might want Ben to suffer, but that’s not the point. The point is that if Ben goes through what he went through they can be a pair again. Anakin wants Obi-Wan to transform. Fire is metaphorically transformation, which is one of the reasons the whole Mustafar scene is so well done. Vulcan is the forge-god, as well as the god of metal. The maker, if you will (c.f. C3-PO’s comment).
What is driving Vader isn’t meanness. What’s driving him is a longing for reunion. With Ben. With Padmé. With his kids. He has only his own transformation by fire to go by, and he thinks transforming those he loves (converting to dark side, anyone?) will complete him. He just does not know what true reunion looks like until Luke shows him, and then it’s clear to him that’s what he was after the whole time.