Here’s my take on the “Must obey” line…
Vader NEEDS Luke to overthrow the Emperor. The Emperor’s hold on Vader is too strong. But, if Luke kills the Emperor, then the two could “rule the galaxy as father and son.” and Vader’s dark hold on Luke would be as strong as the Emperor’s was on him.
This stems from their confrontation in ESB. He’s urging Luke to do what he can’t.
[JEDIT] i may be misremembering how/when that line is used. it’s been awhile since i last saw rotj. but.