Revealing mistake: When the Tydirium shuttle is flying toward the Imperial spaceships, there's a frontal shot of it and a strange thing appears near the shuttle for a few frames. It's not the kind of marks that appears on an old film, or a special effects box. It's like a problem with the matte painting.
Revealing mistake: In the shot where Vader throws the Emperor into the pit, the Emperor is obviously a puppet, because he doesn't move at all. (01:52:15)
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)
Revealing mistake: When Luke somersaults over up to a platform above him when fighting Vader, If you look at the hair of Luke jumping, you will notice that the stunt double's hair is much more blonde than Mark Hamill's.
Revealing mistake: When Luke jumps from Jabba's prisoner's skiff over to another skiff, if you watch Lando hanging underneath the prisoner's skiff, you can easily see that he is a matte painting in this shot, because he doesn't move at all (visible on widescreen).
Revealing mistake: In an overhead shot of Darth Vader throwing the Emperor to his death down the Death Star reactor, you can see that it is a dummy in the Emperor's costume and not Ian McDiarmid.
Revealing mistake: Vader tosses the Emperor and then collapses on some barrels next to the railing. Look at Vader's arm - the one with the severed hand. His arm is bent at the forearm in a way that arms just don't bend. This is because David Prowse's hand was inside an "elongated" arm to simulate it being chopped off. He couldn't keep his wrist straight, giving the appearance of a bent forearm. (01:57:00)
Revealing mistake: When Jabba's sail barge begins to explode, you can see that one of the floating skiffs is standing on stilts. (00:36:30)
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)
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