Some, like the website Dork Side of the Force, believe that Emperor Palpatine’s salvo of Force Lightning is actually what results in the death of Darth Vader. Of course, some might argue that the powerful Dark Side ability wasn’t the smoking gun, but what led to Vader’s death. As anybody who has seen the original “Star Wars” movies might be able to tell you, Vader is incredibly weak when exposed outside of his cybernetic suit. The machinery entombed within the suit and helmet are key to keeping the Sith Lord alive, and a surge of electricity could have fried essential components within the life support systems.
One might also be able to argue that Vader was using his hatred and strong emotions to sustain his broken life, which wouldn’t be the first time a powerful Dark Side user pulled off the feat. In the video game “Star Wars Knight of the Old Republic II: The Sith Lords,” Darth Sion (Louis Mellis) is nothing more than a corpse animated by his strong hatred and skill in the Dark Side. The player is forced to defeat Darth Sion many times before his will finally breaks and the villain dies.
In this regard, when Darth Vader finally gave up on his hatred and was redeemed by his son Luke (Mark Hamill), the Dark Side powers that kept him alive may have subsided. Much like Anakin’s wife Padmé, who gave up on life when Anakin became Darth Vader, perhaps when Vader finally came to terms with his own actions, he was ready to meet his own fate. That would certainly explain his changed visage as a Force Ghost at the end of “Return of the Jedi.”