Continuity mistake: When Bib Fortuna attempts to persuade R2-D2 to show him the message to Jabba, his hands keep changing position between shots. (00:07:20)
Continuity mistake: In the beginning of Luke and Yoda's conversation, there's a cauldron hanging over the flames in the fireplace. In the shots facing Yoda, it is hanging higher up than in the previous shots. (00:28:30)
Continuity mistake: When Lando says goodbye to Han, after Ackbar's speech, Lando's lapel pin/medal swiches shoulders several times. (00:51:40)
Continuity mistake: When Luke and Vader enter the Emperor's throne room, they walk towards the stairs, and as the shot ends, Luke put his foot on the first step. In the next shot, they have suddenly moved further up the stairs. (00:26:45)
Continuity mistake: Just after R2-D2 cuts the net that holds him, Luke, Han, Chewbacca, & C-3PO, you can see that before the Ewoks pop up out of nowhere to see their captured victims, one Ewok just stands there in plain sight waiting for the others to reveal themselves.
Continuity mistake: During the transport to the Sarlacc Pit, in some shots, Chewbacca is standing on Han's right. In other shots, he is standing behind Luke and Han.
Continuity mistake: When Luke and Leia swing to the skiff on the rope, Leia's hands change position between shots twice.
Continuity mistake: On Endor, Leia get shot in the shoulder but when Han finds her, she is wounded lower on her arm.
Continuity mistake: The Millenium Falcon lost its radar in the Death Star, but it is visible at the end.
Continuity mistake: When Luke and Vader enter the Emperor's throne chamber, the stars behind him move really fast. Later, they move in a normal way.
Continuity mistake: Widescreen version: After Leia tells Han that Luke is her brother, she kisses him, and then removes her hand from his neck twice. (01:59:20)
Continuity mistake: On Jabba's sail barge, Luke gets shot in his mechanical hand. This leaves a large burn mark on the back of the hand, but when he swings to the skiff on the rope with Leia, the burn is gone. (00:36:00)
Continuity mistake: In the bunker, just after R2 gets shot, Han tries to hotwire the door. A second blast door closes, and you can see that it is very flat and cardboard looking. A few moments later, Leia gets shot, and as Han is tending to her, notice that the door is now massively armored and three-dimensional, completely different from the door seen earlier.
Continuity mistake: When Han enters the cockpit of the shuttle Tyderium, as the shot ends, he is about to walk past Luke. In the next shot, he has already walked past him and has begun to sit down. (00:50:25)
Continuity mistake: When Vader's shuttle exits the Star Destroyer in the beginning of the film, it is followed by two TIE fighters. The TIE fighters are far apart, but in the next shot, they're close together. (00:02:15)
Continuity mistake: When the Imperials are tricked into leaving the bunker, they are surrounded by Ewoks, and as the shot ends, if you look through the crowd, you can see that Han turns his head to his right. In the next shot, he's facing forward again. (01:46:55)
Continuity mistake: After Han is released from the carbonite, he falls to the ground, soiling his shirt. In the following shots, the amount of dirt changes continuously between shots. Later, when Han and Luke are brought before Jabba, his shirt is clean.
Continuity mistake: In the first shot of the Emperor's force striking Luke, the Emperor's hands are in a different position from the previous shot. This also happens after he says, "You have paid the price for your lack of vision." (01:50:30 - 01:51:10)
Continuity mistake: On the forest moon of Endor, when the stormtrooper punches Han, he falls and has a blaster in his hand but it disappears when he hits the ground. He can't have thrown it during the time he was falling, because he shot accidentally when he was near the ground. If he has thrown it, it should have been near him.
Continuity mistake: When the little Ewok finds Leia, and she rises up, he points his spear at her and you can see that the front of his leather hood is over the spear's shaft. The shot changes to Leia, the back to the Ewok and now spear is over the hood, the shots change again and now the hood's front is crumbled up between the spear and the Ewok´s body.
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