Revealing mistake: In many of the scenes taking place in outer space, if you look closely you can see polygon borders surrounding space vehicles (X-Wings, Star Destroyers, and even the Death Star) indicating the objects being superimposed into the outer space background. [This mistake may only apply to the Blu Ray version.]
Revealing mistake: When Luke is being fed to the Rancor, in Jabba the Hutt's dungeon, there are black outlines around the beast's legs, from the composite's blue screen special effect. This was edited out in the special edition rerelease.
Revealing mistake: When Luke is fighting on a skiff, he kicks one of Jabba's men in the head, causing him to fall off. Luke's foot doesn't make any contact with the person. (00:33:48)
Revealing mistake: Widescreen version only: After Luke cuts off Vader's hand, the Emperor says, "Now, fulfill your destiny, and take your father's place at my side." In the next shot, Luke's lightsaber blade goes through the railing, without damaging it. (01:49:15)
Revealing mistake: When Darth Vader lands on Endor there is a shot of him walking out of the shuttle, underneath the ramp there is slight black spot which probably marked the transition from studio set to matte painting. This was eliminated for the 4K release in 2019. (01:22:47)
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)
Revealing mistake: The shots at the end of the movie with the emperor are obviously lit from the front. This exposed too much of the emperor's head inside the cowl so it was darkened to make him look more menacing. Look for the added shadow - it looks like someone took a black magic marker and coloured on the film. (This may be corrected on the re-release).
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)
Revealing mistake: When Han is running from the explosion of the shield generator on Endor, you can see his reflection in the plexiglass that is used to protect the camera from the explosion. Since the reflection isn't of Han's back, the plexiglass must be in front of him. (01:49:55)
Revealing mistake: When Luke and Darth Vader are fighing in the emperor's throne room, they pass in front of a great window. The stars are blurred and they shake. (01:52:30)
Revealing mistake: When Admiral Piett says "I have orders from the emperor myself," the right edge of the matte can be seen on the right. Theater version only. This has been corrected on the 1997 special edition release.
Revealing mistake: (Original theatrical release) When the imperial officer yells "Freeze", he is transparent and you can see the reactor through him.
Revealing mistake: In many far away shots of the sail barge you can see it is on a dune rather than in mid air.
Revealing mistake: When Luke jumps from Jabba's prisoner's skiff, you can see a stick supporting the "floating" skiff he jumps over to. (00:32:15)
Revealing mistake: When Luke is in his X-Wing preparing to make the jump to Dagobah, you can see the red text in the TV-type viewer in the cockpit has been superimposed as it remains stationary relative to the frame even when the cockpit is moving around.
Revealing mistake: On Jabba's sail barge, Luke's hand is wounded before it is shot at.
Revealing mistake: During the speeder bike chase, the speeders never cast any shadows on the ground.
Revealing mistake: When 3PO and R2 are being assigned to their droidly duties in Jabba's palace, there is a shot of a poor droid having hot irons branding his feet. The irons are never closer than an inch from the droid's feet, and the steam that allegedly comes from the burning is obviously being shot out of holes in the droid's feet.
Revealing mistake: After Wicket tells the rebels where the secret entrance to the base is, there's a shot of spaceships flying in space. An X-Wing passes through the Millennium Falcon and another one through a Calamarian cruiser.
Revealing mistake: Right before Luke leaps from the speeder bike to the scout trooper's speeder, there's a shot showing the front of the speeders. Notice the area surrounding the speeders in this shot. During the rest of the speeder bike chase, the speeders pass through this area several times, even though they are not flying in circles. This can be seen, for example, from the sunlight hitting the ground and the tree on the left side with a bush in front of it. Also, one of the shots where this area is seen is the shot before the speeders pass two more scout troopers, but the background has been flipped.
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