It reminds me that my main issue with the Vader’s plot in ROTJ is the line “I MUST obey my Master”, whereas he explicitly told his son (and his wife !) that he would rather like kill the Emperor and rule the galaxy himself. Just removing that line and leaving only “you don’t know the power of the dark side” would make things work better I guess.
It’s just a slight complain: I know ROTJ is not the same top quality as ANH and TESB and has many problems here and there, but it still is a very good movie (and still the third best SW in my opinion), as a SW and as a fantasy flick.
It is the beginnings of the rule of two. The only way out from under Palpatine is to take his place. That is what Vader suggests. It is kinda the Sith mantra - come with me and we can rule the galaxy together. Palpatine changed it up by telling Anakin that if he followed him that they could save Padme together.