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)
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)
Continuity mistake: When Lando says goodbye to Han, after Ackbar's speech, Lando's lapel pin/medal swiches shoulders several times. (00:51:40)
Audio problem: When the group in the shuttle Tyderium is waiting tensely to see if the clearance code is accepted, Han says, "They're not going for it, Chewie." If you look closely, you can see that when he stops talking, his mouth is still moving. (00:54:00)
Audio problem: After landing on Endor, Han says, "This whole party will be for nothing if they see us.", but his lips hardly move. (00:55:10)
Revealing mistake: If you watch closely (or play it in slow motion) when the scout trooper punches Han, you can see that the troopers hand never actually comes into contact with is face. (00:55:45)
Other mistake: Widescreen version: When Han calls after Luke and Leia when they leave on the speeder bike, the head of the stormtrooper that attacks him can be seen in the lower right hand corner, as the actor is waiting for his cue. After Han calls after Luke and Leia, the stormtrooper suddenly pops up and attacks him. (00:56:15)
Continuity mistake: After the Death Star explodes, in the first shot of the Ewoks, a grey one is standing next to Chewbacca (who can be seen on the extreme right). In the next shot, this Ewok has disappeared, and another one is standing next to C-3PO. This Ewok has disappeared in the following shot, and a third one, who was nowhere to be seen in the previous shot, is now standing in front of Chewbacca. Also, C-3PO, Chewbacca and the Rebel next to him are standing in different positions between shots. (00:58:15)
Continuity mistake: When Leia wakes up in the woods, she sits straight up. In the next shot, she is still lying on the ground. This is best seen in the widescreen version. (01:00:25)
Continuity mistake: When Leia offers the Ewok food, she holds it out in her left hand, but from a different angle it's in her right. (01:01:15)
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)
Continuity mistake: When Wicket sits down next to Leia on the log, he's holding his spear straight up. In the next shot, it is pointing downwards. (01:01:35)
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)
Continuity mistake: After Vader enters the Emperor's throne room to tell him that Luke is one of the Rebels landing on Endor, as the shot ends, he is just about to walk up the stairs. In the next shot, he has already walked up the stairs. (01:03:50)
Revealing mistake: After the heroes get snared and surrounded by Ewoks on Endor, the Ewoks mistake C3PO for a god and start bowing. There is one Ewok which only has holes where the eyes and mouth are and you can see the eyes and mouth of the person inside the Ewok costume. (01:07:50)
Continuity mistake: In the scene where Luke and the others are trapped in the net, the dead animal changes appearance (both injuries and actual appearance) and position several times. For example, the animal seen when they are in the net has a different mouth and a bigger area of white fur from the one seen a few shots earlier. (01:08:35)
Revealing mistake: When Darth Vader lands on the forest moon of Endor, he exits the shuttle and his head passes through it. (01:19:30)
Continuity mistake: After Vader's shuttle lands on Endor, as the AT-AT is walking towards one of the ramps, you can see that the ramps are connected right next to the lift, under the landing pad. But when Vader exits the lift, there's a circular platform going around the lift, which wasn't there in the previous shots. (01:19:40)
Continuity mistake: During Luke and Vader's conversation on Endor, as Vader says, "That name no longer has any meaning for me," he is pointing the hilt of Luke's lightsaber at Luke's chest. In the next shot facing Luke, the hilt is nowhere to be seen, even though it should be, because of its position in the previous shot. (01:20:35)
Continuity mistake: When Vader ignites Luke's lightsaber, Luke is standing next to a column. When Vader extinguishes the lightsaber, Luke is standing further away from the column. In the next shot, he is standing next to it again. This is best seen in the widescreen version. (01:21:10)
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