In Return of the Jedi Vader asks Luke to take his mask off, to which Luke believed he would die. In Empire General Veers walks in on Vader with his mask already off. Given the injuries seen in Revenge weren't entirely related to his respiratory system (and he could still speak so his lungs were OK), what did kill Vader in the end? Luke only chopped his (admittedly robot) hand off, and the Emperor seemed to only graze him with his force lightning. Was his suit damaged? I know his breathing was laboured towards the end.