Continuity mistake: In the fight scenes on the skiffs with Jabba the Hutt's men, the skiff that Han and Chewie are on is hit with a laser blast and Han falls over the edge feet first, yet in the following scene he is hanging upside down with Chewie holding his feet.
Revealing mistake: When Luke somersaults over up to a platform above him when fighting Vader, If you look at the hair of Luke jumping, you will notice that the stunt double's hair is much more blonde than Mark Hamill's.
Continuity mistake: When Han is planting explosives in the bunker, he says, "Throw me another charge." and places the charge on a crossbeam. In the next shot, the charge is on a different part of the ceiling. (01:47:00)
Continuity mistake: In the lightsaber duel between Luke and Darth Vader, Luke chops off Vader's hand twice before Vader falls down. The first shot shows Luke's lightsaber striking Vader's wrist, and Vader's hand separating from the wrist. The second shot is from behind and shows the lightsaber chopping Vader's hand clean off his wrist, then it sparks.
Continuity mistake: When R2-D2 is tied up in the Ewok village, the rope that binds him keeps changing position.
Continuity mistake: After Lando is shot off the transport vehicle when Han is trying to save him, look for Lando to actually walk down the sand in the bottom of the screen. The very next scene, he is hanging on the rope.
Continuity mistake: Just before the Emperor's ship lands on the Death Star, a formation of four TIE fighters disappears from the center right of the frame. This happens seconds before the in-hangar scene with Vader greeting the Emperor. This is visible on VHS.
Continuity mistake: When Lando orders half the fighters to break off and return to the surface, an A-Wing and an X-Wing break off to the right. When Green Leader acknowledges, the same A-Wing and X-Wing break off to the right again.
Other mistake: When R2-D2 cuts the net, only Han, Luke, and Chewbacca fall from it. Maybe C3P0 would have held for a moment because his leg was stuck in the net, but R2 was right over the hole.
Continuity mistake: When Luke takes off Vader's mask, we see Anakin Skywalker with eyebrows on him, but in Revenge of the Sith, Anakin burns on Mustafar and his hair and eyebrows are gone. This has been fixed on the 2004 release.
Revealing mistake: In an overhead shot of Darth Vader throwing the Emperor to his death down the Death Star reactor, you can see that it is a dummy in the Emperor's costume and not Ian McDiarmid.
Continuity mistake: When Luke is riding the speeder bike against the trooper, the black glove on his right hand jumps to his left and then back again. (01:01:15)
Other mistake: During the fight between Luke and Darth Vader, when Vader throws his lightsaber at Luke, it is not only Luke's lightsaber from Episodes IV & V instead of his own, it is ignited upside-down, with the blade protruding from the pommel rather than the emitter. (01:50:20)
Revealing mistake: When Luke jumps from Jabba's prisoner's skiff over to another skiff, if you watch Lando hanging underneath the prisoner's skiff, you can easily see that he is a matte painting in this shot, because he doesn't move at all (visible on widescreen).
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.
Continuity mistake: When Anakin Skywalker (Vader) dies, the path of a tear is visible under one of Luke's eyes. Two shots later, the tear path is dry. (01:55:45)
Revealing mistake: Vader tosses the Emperor and then collapses on some barrels next to the railing. Look at Vader's arm - the one with the severed hand. His arm is bent at the forearm in a way that arms just don't bend. This is because David Prowse's hand was inside an "elongated" arm to simulate it being chopped off. He couldn't keep his wrist straight, giving the appearance of a bent forearm. (01:57:00)
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: In the scenes where the Emperor is trying to convert Luke, watch the stars through the window behind him. The same group of stars go floating past every time.
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