Revealing mistake: Just before Luke chops off Darth Vader's hand in the battle at the end of the movie, you can see that the bar has already been cut (the line is visible) to give the illusion that Luke cut through the steel bar with his lightsaber as well as Vader's hand. (01:48:45)
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)
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: Luke tells Threepio to tell the Ewoks that if our heroes are not set free, that he will use his magic. If you look on the right of the screen, Artoo is standing untied, even though at that point in the movie he had not been untied yet.
Continuity mistake: In this film, the Imperial officers are all wearing the same rank insignia. This was not the case in "Star Wars" and "The Empire Strikes Back".
Continuity mistake: When Lando says, "Fighters coming in," a whole bunch of TIE fighters come towards them. There's initially a lot anyway, but then after about a second, two horizontal lines of 4 fighters appear from nowhere (centre screen, underneath the Super Star Destroyer). They suddenly just pop into existence, large enough that we should have seen them approaching. (01:31:20)
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: 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: During the battle between Luke and Vader, their sabers alternate between casting shadows on the floor, and no shadows. (01:52:40)
Continuity mistake: In the beginning of Luke and Vader's lightsaber duel, and also in a few shots right after Luke knocks Vader down the stairs, Vader's cape alternates between being draped behind and over his left shoulder. (01:48:55)
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)
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 leaves the Death Star in an Imperial shuttle, a large girder or something falls right in front of his ship. If you watch closely, you can see it bounce, revealing the miniature. A huge, heavy girder wouldn't have bounced like this.
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 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)
Revealing mistake: When the shield generator explodes, a piece of shrapnel flies towards the left side of the screen and then flies back, because it bounces off the inside of the building the miniature was housed in. (01:50:05)
Continuity mistake: In the scene where Wicket finds Leia in the woods, they sometimes change position between shots, as seen from the vegetation and the surroundings.
Continuity mistake: In the scene where Luke and the others are trapped in the net, the dead animal changes appearance (both injuries and actual appearance) and position several times. For example, the animal seen when they are in the net has a different mouth and a bigger area of white fur from the one seen a few shots earlier. (01:08:35)
Continuity mistake: When Vader throws his lighsabre, it hits one of the supports holding the catwalk, but in the next shot, it is nowhere to be seen. (01:45:50)
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)
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