Well, it had to do with the final turn. One minute he was happy, nice, and loving, the next minute he was murdering innocent children over an inconsequential dream sequence. Therefore, if a person's fall to the dark side of the force must involve their power to choose their own fate, as the OT implied (and even the PT implied at many points), Anakin must have been psychotic to have made the final choices that he made. If the dark side of the force doesn't involve choices on the other hand, then it operates in contradictory and arbitrary manner. If anyone can be great one minute and pure evil the next simply because the force can illogically force someone to be so, then there is little or no human drama left in the Star Wars universe.