From what I have read, the Emperor thought Luke must suffer for his unforgivable sin of not taking his father's place. So, he wouldn't have suffered as much if the Emperor had just killed him with a powerful bout of lightning. But, when Vader lifts him over his shoulder, he panics and uses his lightning to the maximum, with deadly implications for Vader's mechanical suit.
Re: Question about Darth Vader/Anakin's Death in ROTJ.
Originally posted by Huntlar
Well, when Darth Sidious is using his lightning on Luke for like 20 seconds, Darth Vader picks Darth Sidious up and throws him off that platform thing. It took about 5 seconds to do that, and Darth Vader dies because of it. If he dies, why doesn't Luke?
Well, I'd think it's because he grabbed the source of the electricity itself(Palpatine) and it was far more concentrated this way. Plus, like other posters comented, it really messed with his cybernetic body. I believe those 2 elements alone finished Vader.
Originally posted by OC2-tin-OC
Then how did Vader kill Obi Wan who has learned to fight with the force?
Obi Wan gave up. He sacrificed himself. Vader never "won" this fight.
Originally posted by killphilJust 'cause he was holding his own doesn't declare him the winner, or even make it a draw. He was a weak, out-of-training old man.
That's totally opinion. You might wish that Vader would have won, but you'll never know. Ben was holding his own anyway.
Vader didn't look to hassled either.
I guess you saw the movies in a totally different light than me. I really don't think Vader was all that formidable either. It seems he peaked at ep3 and has only gotten weaker since. Lucas' has said pretty much the same thing regarding that fight, I think he mentions it in the commentary, about how both combatants are merely shadows of their former selves and the fight is a strain for both of them.