Continuity mistake: Look at the full body shot of the holographic Luke. Luke, who normally has his lightsaber clipped to his left hip, now has it hung from his right hip. As well, his hair, which normally has a part on his left hand side, is now parted on the right. Afterwards, just before Artoo turns the message off, we get a shot from behind Artoo, and Luke's hologram is visible on the extreme right of the screen. Luke's lightsaber is now hanging from his left hip where it normally is.
Audio problem: In the desert after Luke starts putting his rescue plan into action, Han says "Boba Fett", before hitting him unintentionally. He immediately says "Boba Fett" again. It is the same sound, looped.
Other mistake: On Endor, after Han Solo's group was captured, C-3PO called out to the Empire's soldiers. One of them said "Bring those two down here!" Both C-3PO and R2-D2 were up there, but from the Imperials were standing, they could only see C-3PO. R2 was hidden behind a tree. They wouldn't know that he was there until they actually got close to the two.
Continuity mistake: When Han is speaking to Lando before he departs for Endor, the distance between the two men changes between shots.
Revealing mistake: The interior matte painting of the hangar on the Rebel ship holding the stolen Imperial shuttle doesn't match the appearance of the hangar from the exterior shot of the Rebel ship. The interior hangar shows an extended platform and four banks of vertical lights bordering the opening and then walls. From the exterior, the hangar appears to have a big light bank near the opening, and then thinner light banks behind it spaced further apart than the interior shot. The exterior shot also doesn't have a platform. (00:50:45 - 00:53:20)
Revealing mistake: When Luke and Leia swing from the deck on to the skiff, you can see that the man playing Lando Calrissian is not Billy Dee Williams but his stunt double. (00:36:25)
Continuity mistake: Before the rebels take off for Endor, Han says to Lando to take the Millennium Falcon as "she's the fastest ship in the fleet". At this point, Lando can be seen moving his head down, but as the scene changes he is looking straight ahead again.
Visible crew/equipment: During Luke's conversation with Ben's ghost, you can see a boom microphone in the background. This applies to the original wide-screen version.
Continuity mistake: As C-3PO and R2-D2 enter Jabba's throne room, C-3PO is walking down a small set of stairs with R2-D2 not following after him. When C-3PO again can be seen after the pillar blocking the camera's view, R2-D2 shows up behind him shortly after. (00:07:55)
Continuity mistake: Outside the bunker the position of the bodies keep changing or moving throughout the battle on Endor.
Continuity mistake: When R2-D2 is shot by an Imperial stormtrooper whilst trying to open the shield generator's doors, he goes into the side wall and you can see a door open on his body with an interface arm sticking out of it. But when Han shoots the stormtrooper dead, we cut back to R2, and the door that was originally open is now closed.
Continuity mistake: When the Rebel fleet is preparing to make the jump to hyperspace to the second Death Star, the small fighters move first. Then the big ships. But when the big ships move into hyperspace, there are only the Medical Frigate, some transports and some Blockade Runners. What happened with the big Correllian command ships?
Continuity mistake: During the speeder chase scene, Leia jumps from her speeder and it blows up, the stormtrooper then hits a tree. The camera shows the tree as a hollow shaft, but when the stormtrooper smashes into it, suddenly it is solid. (01:58:15)
Revealing mistake: The close-up of R2-D2 waiting for Leia to return has a clear shot of the middle wire section of C-3PO in which you can see Anthony Daniels breathing as the spandex underneath the wires expands. This is especially clear on the Blu-ray. (01:02:00)
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