Audio problem: When they are fighting on the smaller skiffs above the Great Pit of Carkoon, Luke beckons to Leia to get her to swing on that rope with him to safety. He says "come on", but his mouth never moves.
Audio problem: When Vader is talking to Moff Jerjerrod in the beginning of the film, he says, "Perhaps I can find new ways to motivate them." As he says the last two words, Jerejerrod's mouth is moving as if he were talking, but nothing is heard. (00:04:00)
Audio problem: After landing on Endor, Han says, "This whole party will be for nothing if they see us.", but his lips hardly move. (00:55:10)
Audio problem: When Lando is inside the Death Star we see a shot of him scraping the side of some metal. Then he says "That was too close." His voice has the same echo applied to it as the sound effects in the narrow metal tunnel. For unknown reasons this mistake wasn't even fixed in the DVD.
Audio problem: When the Death Star is about to explode, Admiral Ackbar says, "Move the fleet away from the Death Star.", but his lips don't move. (01:57:30)
Audio problem: In the beginning of the space battle, a Mon Calamari says, "Admiral, we have enemy ships in Sector 47.", but his lips don't move. (01:31:15)
Audio problem: When the group in the shuttle Tyderium is waiting tensely to see if the clearance code is accepted, Han says, "They're not going for it, Chewie." If you look closely, you can see that when he stops talking, his mouth is still moving. (00:54:00)
Audio problem: After R2-D2 gets shot, Leia says to Han, "I'll cover you.", but the words don't match the movement of her lips. (01:39:00)
Audio problem: When the shuttle Tydirium is approaching Endor, look closely when Han says, "Fly casual" - he begins to say the words before his lips start moving. When he stops talking, his mouth is still moving.
Audio problem: In the desert after Luke starts putting his rescue plan into action, Han says "Boba Fett", before hitting him unintentionally. He immediately says "Boba Fett" again. It is the same sound, looped.
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