Who killed Darth Vader?

Answer 1

Darth Vader, the Sith Lord that was Anakin Skywalker, became so because he surrendered himself to the Dark side based on his feelings of fear, doubt and hate. Thirty years later, when Lord Vader witnessed his son Luke throw down his light saber and refused to fight The Emperor, he also witnessed the true power of the Light side and saw in his own son, the Jedi that he was supposed to become - a powerful, mature, benevolent, fearless master of the Force, (much like his Master Obi-Wan Kenobi.) This demonstration and Luke's faith and continued insistence that Anakin could still be redeemed, is what caused Darth Vader to have respect, compassion, mercy and indeed love for his son in the moments that The Emperor was assaulting Luke with Force Lightning. These feelings were incompatible with and detrimental for the essence of a Sith Lord, but not for the essence of a Jedi. Luke knew this, and knew that the essence of his father still existed inside Darth Vader. Luke demonstrated this faith earlier when he turned his back to Vader during Luke's surrender, allowing Vader the opportunity to decisively end the conflict by killing Luke, who was unarmed while Vader held Luke's Light sabre. Luke's love and faith is was what destroyed Darth Vader and saved Anakin. Anakin in turn saved Luke, sacrificing himself and redeeming himself in the same selfless act. Anakin says to his son, "You've already saved me," and "Tell your sister, you were right, you were right." Darth Vader had already been destroyed when Anakin attacked the Emperor.

Answer 2

The Emperor (A.K.A. the Sith) killed his breathing apparatus when Darth Vader was electrocuted by throwing the Emperor over the edge....literally.