In the original two films, it looks like Vader is slowly gaining power, rather than being the right hand of “The Devil”, as you say. His character is pretty different in ROTJ, he doesn’t do any of the dirty work he used to, he just sulks around. It’s odd to me.
I really like the way Vader was written and portrayed in ROTJ. I would say he is way more complex an interesting compared to the first two films. The way he acts can be explained by two aspects. First, Emperor’s presence and power is reflected through his behaviour and it serves as the primary hint of Emperor’s power. Vader is pretty much established as the ultimate badass and seeing him behave like that establishes the power of the Emperor. Second, even more important, it is great character development. It shows that his encounter with Luke actually changed him and that he is conflicted.