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)
Continuity mistake: When Lando says, "Fighters coming in," a whole bunch of TIE fighters come towards them. There's initially a lot anyway, but then after about a second, two horizontal lines of 4 fighters appear from nowhere (centre screen, underneath the Super Star Destroyer). They suddenly just pop into existence, large enough that we should have seen them approaching. (01:31:20)
Continuity mistake: Widescreen version: When the captured rebels are led out of the bunker, C-3PO walks out from behind a tree. In the next shot, the tree is much bigger, and C-3PO is further away from it. (01:33:10)
Continuity mistake: When the droids are hiding behind a tree before R2-D2 shoots off to help Han and Leia, the background changes twice. (01:35:45)
Continuity mistake: During the battle against the Death Star, right before the Emperor gives permission to fire the Death Star, Wedge says, "Red Three, Red Two, pull in." Another pilot answers, "Got it", but it is neither Red Two nor Three, but Green Leader. (01:36:25)
Continuity mistake: When Vader throws his lighsabre, it hits one of the supports holding the catwalk, but in the next shot, it is nowhere to be seen. (01:45:50)
Continuity mistake: When the Imperials are tricked into leaving the bunker, they are surrounded by Ewoks, and as the shot ends, if you look through the crowd, you can see that Han turns his head to his right. In the next shot, he's facing forward again. (01:46:55)
Continuity mistake: When Han is planting explosives in the bunker, he says, "Throw me another charge." and places the charge on a crossbeam. In the next shot, the charge is on a different part of the ceiling. (01:47:00)
Continuity mistake: When Vader is searching for Luke in the dark, in the first shot, he is walking around a computer terminal, with the great window behind him. In the next shot, he's on the other side of the terminal, with the window in front of him. (01:47:10)
Continuity mistake: When Vader is searching for Luke in the dark, as he says, "If you will not turn to the dark side, then perhaps she will." he is holding what appears to be Luke's lightsaber in his left hand. But when Luke ignites his sabre in the next shot, he somehow has it again. (01:48:10)
Continuity mistake: In the beginning of Luke and Vader's lightsaber duel, and also in a few shots right after Luke knocks Vader down the stairs, Vader's cape alternates between being draped behind and over his left shoulder. (01:48:55)
Continuity mistake: In the first shot of the Emperor's force striking Luke, the Emperor's hands are in a different position from the previous shot. This also happens after he says, "You have paid the price for your lack of vision." (01:50:30 - 01:51:10)
Continuity mistake: In the beginning of the scene where Luke is being electrocuted by the Emperor, the Emperor is standing at the bottom of the stairs. When he says, "Young fool... Only now, at the end, do you understand", he is standing in front of the great window. However, in the next shot of the Emperor, he is standing at the bottom of the stairs again, before he walks down to his previous position. (01:50:50)
Continuity mistake: In the scene of the rebels planting explosives in the bunker, the first shot shows someone with gloves planting an explosive. Then it cuts to a wide shot of Han's team and none of them are wearing that style of glove. (01:51:10)
Continuity mistake: When the Emperor is torturing Luke, in the shot after Luke groans, "Father, please", Vader and the Emperor are standing further away from each other than in the previous and following shots where both are visible. (01:51:20)
Continuity mistake: When Luke pulls Vader away from the edge of the abyss, a wall can be seen behind them, quite close to them. But several times earlier in the movie, there was no such wall in this area of the Emperor's throne room. (01:52:40)
Continuity mistake: During the battle between Luke and Vader, their sabers alternate between casting shadows on the floor, and no shadows. (01:52:40)
Continuity mistake: After Luke cuts off Darth Vader's hand, he turns his head to face the Emperor twice, once in the close-up and once in the wideshot. (01:53:10)
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