First and foremost, the Jedi dont believe in killing their prisoners. This is referenced in many Star Wars works, so is fact. Secondly, Obi-Wan loved Anakin as his own brother, they were basically family, and could no more kill him than someone who was his family. Saying that, Kenobi removed all of Vaders organic apendages, but this was in defense. If he hadn't, Kenobi would have been killed. Thirdly, Vader was left laying unprotected next to a lava flow with no means of escape, and would surely have died were it not for the intervention of Palpatine. Finally, it could be postulated that it wouldn't matter in the long run if Vader lived or died there. Died would have been better, as more people would have survive the Empire, but as injured as he was, and the apprentice to an old and decrep