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 Lando says goodbye to Han, after Ackbar's speech, Lando's lapel pin/medal swiches shoulders several times. (00:51:40)
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: 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.
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: 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.
Revealing mistake: At the beginning of the ground battle on Endor, two stormtroopers are knocked to the ground by the Ewoks. In the following shot, one of the troopers has a big crack in the left shoulder of his armour. The crack was obviously not there in the previous shot. (01:34:40)
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.
Revealing mistake: In the battle on Endor, there is a shot of some Ewoks on a catapult and the at-st fires at them. Notice the second time they get fired at, the blast hits the catapult. Just before the shot ends, notice there is no damage to it. (01:40:10)
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: When Vader throws the lightsaber at Luke and the platform collapses, the point where the platform's columns connect to the ceiling have expanding circles of smoke coming from them as the columns detach from the ceiling, created by the charges used to sever the platform. (01:50:20)
Continuity mistake: Widescreen version: After Leia tells Han that Luke is her brother, she kisses him, and then removes her hand from his neck twice. (01:59:20)
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: 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)
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: When Han enters the cockpit of the shuttle Tyderium, as the shot ends, he is about to walk past Luke. In the next shot, he has already walked past him and has begun to sit down. (00:50:25)
Continuity mistake: When Vader's shuttle exits the Star Destroyer in the beginning of the film, it is followed by two TIE fighters. The TIE fighters are far apart, but in the next shot, they're close together. (00:02:15)
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