Continuity mistake: When Yoda is lying in bed, the creases in the blanket over him keep changing between shots.
Continuity mistake: When the droids enter Jabba's throne room, a three-legged R2-D2 rolls out on the floor. In the next shot, the droid no longer has his third leg deployed.
Continuity mistake: After Luke leaps up to the catwalk during the lightsaber duel, Vader's left shoulder is more covered by his cloak than in the previous shot of him.
Continuity mistake: When Han and Leia kiss after the Death Star explodes, after Wicket shows up, Han's hand is on the back of Leia's head. In the next shot, his hand is on her arm, and Leia's thumb is touching his jaw, which it didn't do in the previous shot. (01:59:30)
Continuity mistake: In the Emperor's throne room, the existence and arrangement of several of the canisters on each side of the lift/elevator changes between shots, several times.
Continuity mistake: When the droids are hiding behind a tree before R2-D2 shoots off to help Han and Leia, the background changes twice. (01:35:45)
Continuity mistake: During the final battle, when some moving ships explode, the fire and the debris suddenly stay stationary.
Continuity mistake: When Leia frees Han from the carbonite, the light shining on the walls and floor keeps changing. For example, watch the light on the right side of the carbonite block.
Continuity mistake: During the battle against the Death Star, right before the Emperor gives permission to fire the Death Star, Wedge says, "Red Three, Red Two, pull in." Another pilot answers, "Got it", but it is neither Red Two nor Three, but Green Leader. (01:36:25)
Continuity mistake: When Chewbacca gives Han a hug in his prison cell in Jabba's palace, he initially puts one arm around him, but as the shot changes both Chewbacca's arms are around Han.
Continuity mistake: When Luke throws a scout trooper off of a speeder bike on Endor, the trooper mysteriously hits a tree back first, but he should have hit the tree face-first. He only rotated across his horizontal axis.
Continuity mistake: After the Ewok takes off on the speeder bike, Han says, "You stay here, we'll take care of this." Leia turns and she can be seen with a very large smile, even showing her teeth. When the camera instantly cuts to a different angle of the scene, Leia is no longer smiling, displaying a deadpan look.
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.
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: 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: 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: 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.
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.
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