Luke repeatedly slips into the Dark Side throughout Return of the Jedi for short moments. This is expanded on in the novels, but, for example: • He Force chokes the Gamorrean guards. • He only offers Jabba two alternatives - give him what he wants or be destroyed. • He attempts to assassinate Jabba with a nearby guard's blaster before falling into the rancor pit. • "It's the last mistake you'll ever make" are his final words to Jabba - a smile plastered across his face. • He gives in to his rage to defeat Vader. Luke, then, is always tempted to use the Dark Side. This is why he's so desperate to avoid the fight with his father. In the end, though, he overcomes it with his mercy and compassion, thereby defeating the Emperor and Darth Vader in one go.