Episode III has not come out yet, so we don't know the exact catalyst or circumstances that lead to Anakin's turn to Vader. However, based on the already released 5 films, we can surmise some of the reasons for his turns. Based on AoTC, you see that Anakin wants power to do good. He also has an "ends justify the means" mentality. I think this desire for power and his "ends justify the means" mentality makes Anakin an easy target for Palpatine to convert into a dark lord. My question is if Anakin turned thinking he could use this power to bring "order" to the galaxy, why would he have stayed w/ Palpatine for so long when it was readily apparent that Palpatine had no interest in making the galaxy a better society in which to live? Was it fear of the Emporer? Was it despair? Did Vader believe that he made a mistake but felt he was stuck w/ it because either he could not defeat the emporer or the forces of good would never accept him back after all he had done?