Trivia: There were a number of takes for the scene in which Jabba the Hutt captures Leia. Because the puppeteers working inside Jabba could not see Carrie Fisher properly, the operator for his tongue accidentally "licked" Carrie's face during several of these takes. Thus in the final footage, Leia turns away from Jabba in disgust the moment his tongue appears.
Trivia: The Rancor at Jabba's Palace was voiced by a Dachshund.
Trivia: Co-producer Robert Watts makes a cameo as the driver thrown out of the Scout Walker by Chewbacca during the battle of Endor.
Trivia: When Weequay falls into the Sarlaac pit during the sail barge assault, a "Wilhelm scream" can be heard.
Trivia: Alan Webb was originally cast in the role of The Emperor, but had to withdraw due to illness.
Trivia: The scene in which an enraged Luke calls his lightsaber into his hand was shot "backwards" - Mark Hamill made himself look upset, entered the shot as if leaving, looked calm, then threw his weapon away. Then the shot was run in reverse.
Trivia: Boba Fett does not have a single line of dialogue in this film. The only sounds he makes are screams when he falls into the Sarlacc pit.
Answer: Darth: "Obi-Wan once thought as you do. You don't know the power of the dark side. I must obey my master." It's implied that the dark side is intoxicating, once you totally give in to the dark side it has a hold on you, and appeals to morality and rationalization are useless against your lust for its power. (On a symbolic level, the dark side is a metaphor for vice. Darth Vader is an addict and abusive parent. It's actually funny how many scenes still make sense if you replace "the dark side" with "the bottle" or similar).
TonyPH
If we bring the prequels into it, it's one of the criticisms of those films that they only make the question of how much Anakin is a "true believer" more confusing. But it stands to reason that at first Anakin may feel vindicated in his resentment toward the Jedi. Later on, Vader may not feel that as strongly, but by then his anger has turned toward himself for failing to save Padme. He may feel that a man as terrible as he does not deserve to be "rescued" from the dark side, leading to a feedback loop where he only gets further enamored with its power and does more evil things which causes him to hate himself even more, and so it goes.
TonyPH