Because you refuse to play second fiddle to ghosts, you forbade him from speaking their names. He stops speaking entirely, maybe he never had anything else to say.
“I should have let you burn,” you tell him.
“Yes, my master”
“Perhaps, I will give you back to the flames”
“If that is your bidding, master”
Even his despair is fleeting these days. A reflex more than anything, leftover energy from a dead man. It reminds you of a snake’s body, moving even after its head has been cut off.
Your underlings quiver in fear of his anger but you know it to be a disappointing echo of Anakin’s (everything in the galaxy is a disappointing echo). It tastes sour, like turned wine on your tongue.
You chain him to the ceiling and make him bleed, make him scream. Afterward he looks at you, exhausted, not sad, betrayed, or angry and says, “It’s not enough, you know.”
You don’t know if he means not enough to stay or not enough to make him leave. Anakin was exquisite in his suffering, this creature….
“This isn’t what I wanted.”
“No,” he answers, “this has nothing to do with that.”
Anakin was a man consumed by desire, but Vader wants nothing at all. He stays, he obeys, because he can’t be bothered to leave, and you made sure there was nowhere for him to go.
That he can’t even be bothered to hate you (hate is saved for Obi-wan, always Obi-wan) is the final straw. You send him away because you cannot bare the sight of him. You send him to Mustafar because you want to wound him. You do not follow for fear there is nothing left to hurt.