In ANH when Obi-Wan gets struck down by Vader he instantly becomes at one with the force and fades away, but in TPM when Maul kills Qui-Gon his body remains and he gets cremated. To make any sense of this you have to momentarily take Obi-Wan?s death out of the picture and focus on Qui-Gon?s and Yoda?s deaths. In ROTJ Yoda passes away of old age and peacefully fades, whereas Qui-Gon dies prematurely at the hands of Maul therefore it can be taken that in order to fade away a Jedi must die of natural causes. So based on this Obi-Wan must?ve died naturally mere moments before Vader?s sabre hit him, but what was it then that killed him? It is my firm belief that he had a premonition through the force that Luke was going to shout out ?Ben? if he lost to Vader and he then died due to sheer disappointment that he didn?t call him Obi-Wan even in his final seconds. Hence Obi-Wan died naturally and disappeared becoming one with the force. Think about it, it makes sense that the once great Jedi Knight Obi-Wan Kenobi just couldn?t take the injustice of being called Ben, it would?ve been the final straw in a long line of crap handed to him by the Skywalker family and the ultimate anti-climax to his life, thus Obi-Wan Kenobi died in a tragic conundrum whereby he died after foreseeing Luke shout ?Ben? which in turn caused Luke to see him die and shout out.