Here’s a bump spurred by the recent discussion of the ‘Obi-wan once thought as you do’ scene for Adywan’s edit.
It always felt like there was more to the story when Vader said ‘You don’t know the power of the Dark Side…I must obey my master.’ What was so powerful that would essentially hold Vader hostage to this evil man? I suppose in the film we are supposed to assume that the Dark Side is so seductive that after you fall you are incapable of ever recovering, but even then why is Vader so adamant about serving Palpatine? It’s not like Palpatine is the source of the Dark Side.
So my theory is that Palpatine has some ability with the Dark Side which Vader cannot replicate, for instance he might have kept Vader alive with the Force after his injuries and may periodically give Vader a new ‘infusion’ to prevent him from further deterioration. In this way Vader is a literal hostage of the Emperor.