At the end of Episode VI: Return of the Jedi, Luke had beaten Darth Vader in a duel, but didn't seem to injure the latter critically — all Luke did was sever Vader's right hand, which was a prosthetic to begin with.
So, why did Vader die after he had thrown the emperor off the deck and saved Luke?
To add to what jwenting said, you can clearly see Vader hit by the Emperor's strikes.
This most probably destroyed a part of his suit which he needs as a survival system as Luke said to him when he wanted to remove his helmet.
Here are some screenshots for the proof of the hits.
A note from @LordApricot giving more information on the 'why':
A number of years ago I read a Star Wars book that was a biography of Darth Vader and he mentioned it there. Here is what the Wookieepedia has to say: "Force lightning required a living conduit for the energy to be channeled; thus Darth Vader, despite being an immensely powerful Sith Lord, was never able to properly employ this ability, as any attempt to generate it carried the risk of shorting out his life support systems."
Vader was mortally wounded by the blasts of energy released during the fight. He was hit multiple times by the Emperor.
As he depended on his suit for survival, and it's conceivable the systems in that suit could become overloaded by that energy release, he could possibly be killed by blasts that the healthier, younger, Skywalker could (barely) survive.
Vader's suit was a life support suit - he couldn't survive outside of it.
The long combat with Luke pushed him towards his physical limits - he is at least 20 years older than Luke, and with very poor health. He'd been drawing heavily on the Force to keep himself moving.
He was injured - the suit was damaged when the cybernetics in his right arm were damaged.
THEN he suffered the extreme emotional turmoil of seeing his son being tortured to death.
When he intervened to stop it (by killing the person who had been his mentor for 20+ years, who had saved his life, and who had helped him save the galaxy - from his POV, at least) he was subjected to several seconds of sustained Force Lightning.
This played merry hell with his already-damaged suit.
The man was beaten into the ground, had to choose between his greatest friend's life and his son's, was electrocuted, and had decades-old mental wounds torn open.
It's not surprising that he died, it's surprising that he lived as long as he did.
"Did you ever hear the tale of Darth Plagueis?" The Supreme chancellor (Palpatine) asks Anakin Skywalker this question during the civic performance from his VIP box seat? Thus, preempting a response by Anakin. This is turn sets the stage for the story of a Sith Lord that was so powerful he could even escape death.
Certainly, the suit allowed Vader to live and it's beyond dispute the suit was damaged from the Emperor's force lighting assault on Luke that also rippled through Vader. But I return to Palpatine's original question from SW III and the dark side power his story revealed - the ability to escape even death, and "if we work together (to save Padme)..." These are vital dialogue clues.
I believe Vader's death is not as simple as some would argue. The Force never is. Thus, the suit failure was not catastrophic. It was the Emperor's and Vader's dark side force powers that allowed Vader to thrive from the knowledge of Darth Plagueis - "If we work together"...
Once the Emperor was removed from the equation, Vader's death was inevitable... Vader understood this all too well. It also explains why, after 20 plus years, Vader never slew Palpatine, and became the Emperor himself, which is what Sith do - if anyone has studied their history?
This is why Vader told Luke it was too late. If it were only a matter of the suits mechanical failure though, there would have been time for Luke to get him medical aid by leaving the mask on. He was still laboring with breath. But there is more to it than that.
It had been the dark side of the force that sustained Vader through Palpatine all along. With Vader's sudden disconnection to such power, as well as his turning back to the light side (meaning Vader would not invoke dark side force energy to sustain himself), Vader simply chose to let go... He had found his true peace.
Thus, Vader/Anakin allowed the will of the force to prevail, and in the process fulfilled the prophecy of bringing balance to the force not in a way foreseen by either Jedi or Sith.
In conjunction with the life support suit it was probably Anakin's hatred and anger fueling his Sith abilities that allowed him to sustain himself from the end of Eps3 through to Eps6. As he remembered what it was to be a true Jedi by interacting with his son (as early as tESB) that anger began to subside, compromising his Dark Side ability. When he finally died it was likely because he had made peace with himself and simply stopped wanting to live.
Darth Vader is one of the mightiest Force-users in the entire series. But for all of his power, there is one thing we never ever see Darth Vader do, even in the Expanded Universe: he never once uses Force Lightning. In some cases, like the video games, his lack of this power is conspicuous in its absence. Other users of the Dark Side apply it liberally, but never him.
Why not? According to the Revenge of the Sith Visual Dictionary, the problem lay in Vader's cybernetics and armor. Essentially, his armor's electronics were not well-insulated, and thus extremely vulnerable to this power. Even if Vader were to direct the lightning at another person, there was too much of a risk that a stray arc could destroy his life-support systems. In the EU it is clear that he must have learned the basics of this power at some point, because he went on to teach it to his secret apprentice, but he never used it himself.
And this, ultimately, is what sealed his fate in Return of the Jedi. Whether or not he received a dose of Force Lightning that "should" have been lethal barely even matters, because the suit's life-support systems gave out. Even before his mask is removed, you can hear the difference in his breathing: he can gasp in a very little bit of air, but they're not the full, deep breaths you heard before. That's not enough to stay alive for very long, and even this assumes that only the respiratory functions of his armor were the only life-support systems that gave out, which isn't very likely. In short, even if nothing else had happened to him, this would have killed him.
That he stayed alive (and conscious) for as long as he did is possibly best hand-waved as him using the Force to remain among the living just long enough to conduct his final business: reconciling with his son. When that was finished, he let go.
According to the official novelisation, Darth suffers a number of fairly serious injuries in his fight with Luke and the Emperor, notably a heavy blow to the head and force-lightning bolts to his heart.
Note that while the removal of his life support helmet was the proximate cause of death, the novel makes it clear that he was already dying at the point that this occurred:
When Luke pushed Vader back to break the clinch, the Dark Lord hit his head on an overhanging beam in the cramped space. He stumbled backward even farther, out of the low-hanging area. Luke pursued him relentlessly.
Ignoring pain, ignoring his shame and his weaknesses, ignoring the bone-crushing noise in his head, he focused solely and sightlessly on his will - his will to defeat the evil embodied in the Emperor.
Palpatine struggled in the grip of Vader’s unfeeling embrace, his hands still shooting bolts of malign energy out in all directions. In his wild flailing, the lightning ripped across the room, tearing into Vader. The Dark Lord fell again, electric currents crackling down his helmet, over his cape, into his heart.
Vader motioned Luke closer to him. ‘Luke, help me take this mask off.’
Luke shook his head. ‘You’ll die.’
The Dark Lord’s voice was weary. ‘Nothing can stop that now. Just once let me face you without it. Let me look on you with my own eyes.’
...
Vader, too, was afraid - to let his son see him, to remove this armored mask that had been between them so long. The black, armored mask that had been his only means of existing for over twenty years. It had been his voice, and his breath, and his invisibility - his shield against all human contact. But now he would remove it; for he would see his son before he died.
This is also backed up by the film's junior novelisation:
From where he lay, Luke could tell by the rasping rattle from Vader’s helmet that his breathing apparatus was broken. Luke crawled the short distance to his father’s side and pulled him away from the edge of the abyss.
Anakin Skywalker died the same way Mace Windu did, by powerful blasts of Palpatine's lightning. Palpatine was slowly building up to a lethal intensity while slowly torturing Luke. While his suit probably was damaged by the lightning, no one could have survived a direct hit by that lightning, as Mace Windu did not. Anakin's insides lit up on contact with the bolts, and not even for a second did that happen to Luke. Suit or not, he was mortally wounded, as the remaining tissue in his body was fatally damaged.
Nothing else contributed. It was all Palpatine's lightning.
As most others have already correctly mentioned.. vader suit was shorted out by Palpatines force lightning. Since Vaders breathing is machine controlled and because he needs to be in a pressurized chamber at all times in order for his greatly damaged lungs to be able to absorb enough oxygen to keep his biological cells functioning.. once that system is compromised, he is in real trouble.
That is why when Vader told Luke to "Help me take this mask off" Luke said "But you'll die".
As another mentioned, vader was not in good health due to his injuries, he was older, though that doesn't make any real difference to most Jedi, he hand was already injured from being cut off by Luke and also, in the book version of Return of the Jedi, Vader severly slammed his head against the gangplank that he cut down when he threw his saber at Luke earlier. When Luke came after him so ferociously later on.. that is when Vader slammed his head hard. Even though the helmet protected him from some of the impact, he still suffered what was basically seemed to be a severe concussion which could be why he simply had no more energy when Luke was batting him hard with the lightsaber.
All of these things, combined with getting hit by the lightning bolts are what eventually did him in.
Hate was the only thing that kept Vader alive from his injuries. Hate of everything; Kenobi, himself, Sidious, the Jedi, etc. Hate and fear made him cling to life. When he felt love and attachment to Luke, he let go his hate and fear of death. When he let go of his hate, he let go of what was keeping him alive and he was ok to just let go. Vader's main problem was in not letting go and having attachments. He would not even let go of life, despite his injuries and losses. Hat can be a great fuel and motivator. When he let go of his hate and hurt, he just let go all the way. Sometimes a person loses willpower through love and acceptance.
Sidious knew the weaknesses of "the suit" which is one of the main reasons why Vader never confronted him even though Vader made countless modifications to his suit and armor, remember Anakin although a Jedi was a master mechanic. My ultimate answer to Vader death is the same as Padame's. He simply lost the will to live. Darth Vader is more complex then we'll ever know, I believe that his wrong doings scarred and haunted the Sith Lord ever since the Jedi Purge. Moreover, Anakin was well known for his ability to withstand great amounts of pain. Therefore, Vader being electrocuted by Sidious is no more than him being burned alive in Mustafar. His suit may have been damaged after the force lightning attack but not severely. In end, this is how the prophecy had to end and my main belief that he "lost the will to live".
May the force be with you -Lance
You are all missing something, remember when the Emperor was thrown over the rail
and into the shaft Luke crawled over to Vader and his breathing was VERY labored,
someone nailed it on the head, when he picked the emperor up he was absorbing the lightning
as Obi-Wan stated
"He's more machine now than man"
Machines and out of control electricity don't mix, the lightning probably blew every circuit board in his life support, I'm willing to bet the only thing that was working in his system may have been the vocals and the optics...
My theory anyway ...:)
I decided to look up this myself to see what other people thought about the death of Darth Vader, and here's what I believed is what happened.
As you notice when Anakin was found burnt to a crisp with only 1 limb remaining he was given a breathing mask whilst being carried off the planet. Not only would his exterior body have been severely burned but it's also likely that his lungs were badly damaged from breathing in the hot air and ashes when his clothes set on fire and being so close to the lava lake. When people are pulled out of house fires they're immediately given an oxygen mask to help them breathe so their lungs aren't further damaged from breathing in hot air and ashes.
The suit appeared to be giving off gases, possibly those used in cryogenics, as well as providing him with oxygen. My guess is that the suit was keeping his body from warming up so that he wouldn't die of shock from the severe burns he sustained (pain from such burns can be excruciating if the skin doesn't repair itself properly and can last a lifetime and be very sensitive to exposure and touch. Severe burns cases often die from the shock. Cooling the skin reduces the chances of burns from advancing, which is why I believe Vader's suit was cryogenic but not so much that it would put him into hibernation.)
The suit being damaged would've shut off all life support and his already damaged body was receiving full damage from force lightning when he threw the emperor off the bridge, which would've caused severe pain all over and resulting in him going into shock.
He wasn't that old himself, maybe around 40 years old give or take, so being older didn't really make a difference to him receiving damage from force lightning. It was just that his body was too badly damaged and incapable of surviving such a powerful blow.
After sustaining his life threatening injuries on Mustafar; Vader's life support systems on his suit were the only thing that kept him alive. With the suit damaged beyond repair by the Emperor's force lightning it was inevitable that he would die. Even if Luke had managed to defeat the Emperor and turn Anakin Skywalker (A.K.A Darth Vader) back to the light side of the force, Anakin Skywalker would have been taken arrested by the Rebel Alliance as a prisoner of war. Anakin Skywalker then would have been found guilty of genocide, and crimes against the galaxy and later executed. So Anakin dying on board the second death star happened for the better good.
The reason why Anakin died was that he absorbed lightning through his prosthetics which mortally wounded his organic body and so fried him that it made his bones glow. After absorbing the lightning, it fried all his circuits and his machinery. The real reason why Anakin died was because when he inhaled normal oxygen, it suffocated him and killed him. He was not supposed to inhale normal oxygen but he was supposed to inhale medicated oxygen. See the difference?
Short answer - Palpatine forced him to die.
The Emperor may have been torturing Luke with weak blasts but when Vader attacked him he would have tried and probably succeeded in increasing the intensity.
And powerful Siths probably have the ability to think people to death without any visible effects. The Emperor was probably attacking Vader with invisible force attacks while also blasting him with force lightning.
And after the Emperor apparently exploded far down the shaft, some strange type of wind or energy seemed to come rushing back up the shaft and past Vader. No doubt whatever it was would have been bad for one's health, and especially bad for Vader's.
Anakin was burnt to a crisp on Mustafar and survived for an extended period of time before Palpatine rescued him. Further evidence suggests that the Empire relied on Palpatine's extreme charisma and Force power to remain cohesive.
The death of Palpatine broke the Imperial soldier's cohesion and they were no longer an effective fighting force. My belief is that the sudden overload of force electricity that would have killed anyone else. The Emperor was torturing Luke attempting to turn him to the dark side and then realized his resolve was too strong and was going to finish him. Vader was preparing to kill Palpatine anyway and Vader was fully capable of healing himself for a short period of time as exhibited in TESB When the officer walks into his meditation chamber.
In the expanded universe Starkiller unmasked Vader and he was fine, he just had a harder time breathing. All he needed were prosthetics
Vader was injured from Luke but the real reason was that he picked up the emporer who was lightning Luke the emporers fingers went up surprised, the lighning shot at Vader since his suit need electricity to survive that was giving him a lot maybe a little too much damaging his mechanical organs from his breathing to his blood flow Vader gave up hope knowing he wouldn't survive
