First timer here, just thought i'd put in my two cents. From rumor that i've heard I believe that there is some big arguement between Obi Wan and Anakin that turns Anakin. I've also heard that at some point, my guess is obviously after he turns, there is a big fight between the two. Obi Wan woops Anakin and pretty much cuts him to pieces, thereby forcing Anakin to become half machine as is seen in ROTJ.