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 Vader's shuttle is approaching the Death Star, the crew in the control room of the Death Star are wearing black gloves. One of them then reaches forward to press a button, but in the close-up of his hand pressing the button, the hand is not wearing gloves at all. This can only be seen in the widescreen version. (00:02:25)
Continuity mistake: When Vader's shuttle has landed inside the Death Star, behind the stormtroopers on the left side, there is a large structure. But when Vader and Jerjerrod, the Death Star commander, pass these stormtroopers, the structure is no longer there. (00:03:20 - 00:04:05)
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)
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: As C-3PO and R2-D2 enter Jabba's throne room, C-3PO is walking down a small set of stairs with R2-D2 not following after him. When C-3PO again can be seen after the pillar blocking the camera's view, R2-D2 shows up behind him shortly after. (00:07:55)
Continuity mistake: When the droids enter Jabba's throne room, in the shot facing the droids, they are not standing on the same part of the floor as in the previous shot. Also, R2-D2 is facing Jabba, but in the previous shot, he was facing C-3PO. (00:08:15)
Continuity mistake: When R2-D2 is assigned to his duties in Jabba's palace, another droid (EV-9D9) says, "But you'll soon learn some respect." and her arm is in a different position from the previous shot. In the following shot, her arm is in its original position. (00:11:10)
Continuity mistake: In the Special Edition, Sy Snootles (the lead singer of Jabba's band) is CGI, but in the original version, she was a puppet, looking a bit differently. In the first shot of the band sequence, you can see the old puppet singing on the stage. (00:11:50)
Continuity mistake: When Boussh (Leia in disguise) is about to enter Jabba's throne room, there's a shot of Jabba where his arm is down by his side. In the next shot, on the left side of the screen, his arm is up by his head. (00:13:50)
Character mistake: As they take Han away, watch Lando. Just before he tilts his mask down so we can see who he is, he whacks his head on the doorway. (00:16:20)
Continuity mistake: When the guards take Chewie to his cell, you can plainly see that when the guard closes the door you only hear it close. You can see that it is still open. (00:17:15)
Continuity mistake: When the carbonite block begins to lower to the ground, Leia is standing in a corner. In the next shot, she is much closer to the corner. (00:17:40)
Continuity mistake: After Leia frees Han from the carbonite, she begins to put her hand on his face as she kisses him. As the first shot ends, her hand is on his jaw, but in the next shot, her hand is on his cheek. (00:19:10)
Continuity mistake: When Leia gets shoved on Jabba's belly upon her capture, watch carefully - in the side shot of her being pressed against him, we see her arching back from her master-to-be as he talks. Their faces are a good foot or so apart. However, in the next shot of her, we see her face is mere inches away from him as he licks his lips in horny anticipation. (00:19:54)
Continuity mistake: In the first shot of Chewbacca when he is in Jabba's dungeon with Han, his bandoleer is on the wrong side. (00:21:25)
Continuity mistake: When Luke confronts Jabba the Hutt, Jabba says to Bib Fortuna, "You weak-minded fool." In the next shot, Jabba's tail has switched position. (00:22:55)
Continuity mistake: When the Rancour grabs Luke, as the shot ends, Luke is holding the bone with two hands. In the next shot, he's holding it with only one hand. (00:25:20)
Continuity mistake: When Luke wedges the bone into the Rancour's mouth, in the first shoot from behind, he holds the bone horizontally. In the beginning of the next shot, it is pointing upwards. (00:25:25)
Continuity mistake: In the Rancor pit, watch as the Rancor starts to pick up the pig guard. The Rancor starts to grab the pig guard with its left hand. But while being eaten the pig guard is held in the Rancor's right hand. (00:25:40)
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