I must say though if this episode had come out first, it would have been quite a treat to see the "chosen one" descent into madness. So in order to leave everything pristine for the original trilogy you would just end it at Mustafar where Obi Wan sees Anakin "die" so by the next picture we just assume Darth Vader is just another Sith lord recruited by Palpatine just like good old Darth Maul. If done correctly, you could have surprised your audience twice.