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: After Luke kills the Rancour, Jabba says, "Bring me Solo and the Wookiee", and Lando is nowhere to be seen. In the next shot, he is suddenly standing in front of Jabba's platform, and some of the other people have changed position. (00:27:00)
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 Jabba pulls Leia's chain, there is a long shot as she leaps up onto his throne, and he is holding his wine goblet in his left hand, and pulling her chain with his right. However, in the very next shot (in which he says "Soon you will learn to appreciate me"), the entire frame is reversed. This is evident in Jabba's holding the wine in his "right" hand, his higher-placed nostril (which is normally on the left on his face) is now on the "right," Leia's reversal of her armlet, etc. (00:29:20)
Continuity mistake: When Luke is to be executed, he walks to the edge of the plank, and his feet are side by side as he stops walking. But in the beginning of the next shot, one of his feet is behind the other and we see him take another step. (00:30:55)
Continuity mistake: When Luke is prodded out to 'walk the plank', into the Sarlaac pit, he jumps off the board and springs himself back up. If you watch this carefully, you can see that only one hand catches the board and that just by the fingertips, but in the next shot both hands are springing him upwards. (00:31:20)
Revealing mistake: When Luke jumps from Jabba's prisoner's skiff, you can see a stick supporting the "floating" skiff he jumps over to. (00:32:15)
Revealing mistake: In the wide-shot where Luke leaps from the prisoner skiff over to another skiff, you can see the stick used during the filming, which is much shorter than the regular one. The blade is not even glowing. (It is not just simply the hilt of the sabre, it's too long for that.). (00:32:15)
Continuity mistake: When the prisoner skiff is struck by laserfire, Han Solo is knocked overboard. A long shot shows him falling over sideways, yet when it changes to above the Sarlacc he is pointing straight downwards with his feet caught in the metalwork. (00:33:10)
Continuity mistake: When Boba Fett falls into the mouth of the Sarlaac, he disappears just before the beak touches him. (00:33:45)
Revealing mistake: When Luke is fighting on a skiff, he kicks one of Jabba's men in the head, causing him to fall off. Luke's foot doesn't make any contact with the person. (00:33:48)
Continuity mistake: When the Sarlacc tries to drag Lando into the pit, the tentacle is grabbing his ankle in some shots. In other shots, it is wrapped around his thigh as well. (00:33:50)
Continuity mistake: When Luke and Leia land on the skiff after swinging on the rope, Luke's facial hair is much darker than in the rest of the scene. (00:35:00)
Revealing mistake: When Luke and Leia swing to the skiff on the rope, in the shot from the back, Leia is obviously played by a stunt woman - the birthmark on Carrie Fisher's back, which can be seen a few times on board the sail barge, is missing. (00:35:00)
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)
Revealing mistake: When Luke and Leia swing from the deck on to the skiff, you can see that the man playing Lando Calrissian is not Billy Dee Williams but his stunt double. (00:36:25)
Revealing mistake: When Jabba's sail barge begins to explode, you can see that one of the floating skiffs is standing on stilts. (00:36:30)
Continuity mistake: At the end of the scene when the Emperor and Darth Vader walks through the Death Star's hangar, you can see that the Emperor walks past a squad of grey suited commanders. But in the next shot there are only white dressed storm troopers and scout troopers on the right side of the Emperor. And if you compare the last shot in the scene with the earlier shots you can see differences in the soldiers' placements. (00:37:45)
Revealing mistake: In the scene where Han and Lando are talking in the hangar after the big meeting about the new Death Star, right when the scene opens and the shot pans down, notice how half the hangar is real and the other half is a matte painting. (00:49:30)
Continuity mistake: When Han tells Lando that he can fly the Millennium Falcon during the battle, when Lando says, "Would you get going, you pirate", his entire outfit is reversed - his shoulder holster strap goes the other way, and his rank insignia changes side too. (00:50:00)
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