Lucas acknowledged in the ROTJ story conferences that he was basically jettisoning Vader’s old motivation of wanting to overthrow the Emperor and rule alongside Luke.
Which makes sense, for the dramatic needs of the film - Vader as a seemingly helpless puppet of the Emperor who doesn’t realize he can kill his master is much more amenable to redemption than a Vader who craves power and merely thinks the wrong guy is in charge.
But it does create a discontinuity with the more personally ambitious, selfish Vader in ESB. I suppose with the idea during ESB’s filming that the third film would feed into a Sequel Trilogy with Luke’s lost sister, Vader’s redemption wasn’t a pre-determined part of the saga plotline. It still could’ve happened, but it wasn’t a necessary dramatic keystone.
In which case it’s possible to imagine a Vader who remains evil and selfish, and a Luke who won’t kill his father but lets him die from a perilous situation. Like Batman letting Ra’s Al Ghul die in Batman Begins - or a robot “letting a human come to harm through inaction”, in the phrasing of Asimov’s Laws of Robotics.