The action immediately follows Luke Skywalker’s unmasking of his father, Darth Vader, as seen in the movie.
«Luke takes his mask off,» Lucas told his co-writer Lawrence Kasdan (via a transcription of the meeting from io9).
«The mask is the very last thing – and then Luke puts it on and says, ‘Now I am Vader’. Surprise! The ultimate twist. ‘Now I will go and kill the [Rebel] fleet and I will rule the universe’.»
Kasdan responded, «That’s what I think should happen», but Lucas backed down from the idea, insisting that the film «is for kids».
And we thought that Star Wars: The Force Awakens‘s revelation that Return of the Jedi was followed by divorce, betrayal and abandonment was grim…
The idea of Luke turning to the dark side was later explored in the popular 1991 comic Dark Empire, where he was briefly the apprentice to a resurrected Emperor Palpatine. But we were spared any stories of him massacring all his friends.