I think what ShamanWhill is talking about actually is what’s happening in RotJ.
Vader can’t kill the Emperor. The Emperor’s hold is too strong on him. Vader needs Luke to do it, which is his real reason to bring Luke to the Emperor. Vader knows that the murder will make him the new Emperor & make Luke much easier to turn to the darkside, where they can “rule the galaxy as father & son.”
I do wish this was mentioned in their scene at the docks though…
Seeing his son tortured is what finally gives Vader the power to break the darkside hold & do the deed himself. And thus the Return of the Jedi & a silent title-drop.Hmm… But then again, Vader defends the Emperor when Luke picks up his lightsaber.
It’s not perfect, but I chalk it up to the Darkside’s grip. If Vader “must obey (his) master.” this would include protecting him, willingly or not.
And even the Emperor asks Luke to kill him.
The Emperor wants Luke to kill Vader so that he can inherit a new, younger minion.
Both baddies are working against each other.