Visible crew/equipment: When C-3PO And R2-D2 come up to Jabba's palace in the beginning of the movie, in the close-up, you see overhead stagelights in the reflection of C-3PO's head.
Continuity mistake: When Lando is dangling from a rope over the Saarlac pit, he keeps moving up and down the rope between shots.
Revealing mistake: During the lightsaber duel, Vader throws his at Luke. The catwalk behind Luke explodes, but if you watch closely there are two explosions: one some inches over where the lightsaber actually hits and one far on the left.
Other mistake: When Leia gets on the speeder bike, it starts before she pushes the button.
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".
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 scene where Wicket finds Leia in the woods, they sometimes change position between shots, as seen from the vegetation and the surroundings.
Continuity mistake: Just after R2-D2 cuts the net that holds him, Luke, Han, Chewbacca, & C-3PO, you can see that before the Ewoks pop up out of nowhere to see their captured victims, one Ewok just stands there in plain sight waiting for the others to reveal themselves.
Continuity mistake: During the transport to the Sarlacc Pit, in some shots, Chewbacca is standing on Han's right. In other shots, he is standing behind Luke and Han.
Continuity mistake: When Luke and Leia swing to the skiff on the rope, Leia's hands change position between shots twice.
Continuity mistake: On Endor, Leia get shot in the shoulder but when Han finds her, she is wounded lower on her arm.
Continuity mistake: The Millenium Falcon lost its radar in the Death Star, but it is visible at the end.
Continuity mistake: When Luke and Vader enter the Emperor's throne chamber, the stars behind him move really fast. Later, they move in a normal way.
Revealing mistake: During the speeder bike chase scenes, speeders bikes don't affect any vegetation, even though they're flying near the ground at incredible speed.
Continuity mistake: In the bunker, just after R2 gets shot, Han tries to hotwire the door. A second blast door closes, and you can see that it is very flat and cardboard looking. A few moments later, Leia gets shot, and as Han is tending to her, notice that the door is now massively armored and three-dimensional, completely different from the door seen earlier.
Visible crew/equipment: During the speederbike chase, there are several shots where the support arms holding up the fronts of the bikes are visible.
Continuity mistake: After Han is released from the carbonite, he falls to the ground, soiling his shirt. In the following shots, the amount of dirt changes continuously between shots. Later, when Han and Luke are brought before Jabba, his shirt is clean.
Continuity mistake: On the forest moon of Endor, when the stormtrooper punches Han, he falls and has a blaster in his hand but it disappears when he hits the ground. He can't have thrown it during the time he was falling, because he shot accidentally when he was near the ground. If he has thrown it, it should have been near him.
Revealing mistake: When Leia takes a speeder to chase the stormtrooopers, there is an SFX box around it and it makes no shadow on the ground.
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