I think the 'turn' would have been more convincing if Anakin had been 'expelled' or at least 'suspended' from the Jedi Order early in episode 3, upon his marriage to Padme being revealed/exposed. it really could have been done without effecting too much of the rest of the events in the film and could've driven a greater wedge between Anakin and Obi Wan and pushed Anakin closer to Palpatine/sidious.