Why did Darth Vader die in Return of the Jedi?
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?
I just finished my annual Star Wars marathon and picked up your thread. All of the answers are so well thought. I noted to myself that Vader's suit malfuctions before his hand is re-amputated. He falters, weakens and, yes, after he is disarmed (literally), can barely stand even though he did not suffer a mortal wound during his duel with Luke.
It's entirely possible that he (Darth Vader) simply fell unconscious due to the combined pain of Sith lightning and his suit shutting down. Although I'm pretty sure he wouldn't have survived the Death Star exploding.
as a complete sidenote, when my bro & me played with SW-figures, we had this notion that Luke brought Darth to the ship and connected him to life-support, thereby saving his life. The fire-burning part was just burning the old shell. That made Vader a good guy, alive & kicking :D
The Force lightning and whatever the hell that blue stuff coming out of the pit he threw Palpatine into killed Vader. And he was an old half robotic triple amputee with half his organs missing anyway.
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."
You've made me curious now; were those skeletal shots in the original moves, or were those something added in when Lucas "revamped" them back whenever that was?
@eidylon From what I remember, they were in the original ones but it seems they have been enhanced (and some of them added as I don't remember they were so many occurrences of this X-Ray style effect).
I have the original VHS (I just dated my self I know) and the "skeletal shots" and the "X-ray effect" are in them just not as brilliantly colored/lit.
The skeletal shots are mostly flashes; they're subtler in the movies than in these screen shots because they're interspersed with general flashes of light and the black mask
Also the fact that his prosthetic arms made him incapable of making force lightning suggests that he is even more vulnerable to electricity than a normal person.
@ThePopMachine A number of years ago a read a star wars book that was a biography of darth vader and he mentioned it there. Here is what the wookipedia 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."
@LordApricot: That's not my point. Being incapable of making force lightning doesn't imply he's "more vulnerable to electricity than a normal person" at all.
@eidylon - the skeletal images were present in the original release of the movie, but were updated and revised for the remastered versions.
An additional piece of evidence is that immediately following this, Vader's breathing sounds change. They become gaspy. We can't say whether that's due to injury or equipment failure, but it's pretty clearly a sign something has gone wrong.
You would include the quote from @LordApricot in the answer, it specifically states that his suit is vulnerable to force lightning.