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)
Other mistake: Widescreen version: When Luke and Vader enter the Emperor's throne room, the stars seen through the window behind the Emperor move a little faster than the stars seen through the two other windows. (01:26:35)
Other mistake: When the ground battle on Endor begins, Leia runs to the bunker, and as the doors close, she fires a shot. In the original version of the movie, the blaster smokes, but no laser beam comes out of it and no sound is heard (do not confuse this with the sound of the doors closing). In the Special Edition, the sound has been added, but the laser is still missing. (01:38:05)
Other mistake: After the catwalk that Luke is standing on collapses, Vader walks down the stairs to find him. In the first shot, there's a shadow of a lightsaber blade on the right side of the steps, but Vader's sabre is extinguished at this point. It can't be the shadow of Luke's sabre. Widescreen version only. (01:46:00)
Other mistake: During the fight between Luke and Darth Vader, when Vader throws his lightsaber at Luke, it is not only Luke's lightsaber from Episodes IV & V instead of his own, it is ignited upside-down, with the blade protruding from the pommel rather than the emitter. (01:50:20)
Other mistake: DVD and Blu-ray only: in the Battle of Endor, right after the rebel fighter says "there's too many of them." The Millennium Falcon goes into 1/4 of the picture, you can see a black square spot on the right side of the flash frame due to a bad filter usage. Widescreen only. It does not appear in the 4K release even though it wasn't removed.
Other mistake: When Vader first arrives at the station, he and the Commander are walking past rows of troops. If you look carefully, you will see a Stormtrooper in the back row with his helmet tipped so far forward it's almost off.
Other mistake: During the final confrontation with Vader on the Death Star, Luke bashes a catwalk railing with his light saber several times, but there are no burns, marks, or indentations on it at all. Considering the power of a light saber (Qui Gon melted a steel door in Phantom Menace), some damage should be visible. Only when he hacks off Vader's hand does the light saber slice through the rail.
Other mistake: In the original cinema release, one of the subtitles for Jabba's conversation is incorrectly spelled - 'there will be no bargin'. This was corrected for the video release.
Other mistake: When Vader's shuttle lands on the forest moon of Endor, an Imperial AT-AT walks near the landing pad. There's a strange shadow that keeps appearing and disappearing on one of its leg. Maybe the shadow of the puppeteers?
Other mistake: When Leia gets on the speeder bike, it starts before she pushes the button.
Other mistake: Look at the shots of the windows in the Emperor's chamber on the Death Star. When they are seen from a distance, the window frames are out of focus, but the stars remain in clear focus.
Other mistake: When R2-D2 cuts the net, only Han, Luke, and Chewbacca fall from it. Maybe C3P0 would have held for a moment because his leg was stuck in the net, but R2 was right over the hole.
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.
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