I would like to speculate the logic behind the Star Wars Prequels here for a moment, just to see how close I am a week from now:
Part of this, I would think, is helped by the concept of them being prequels and having an established destiny they need to fulfill. Anakin had to become evil because future films (original trilogy) required him to do so. But taking this a step farther, the destiny was vague, saying that Anakin would only bring great change to the force. It never said for good or ill. And in his case, it was both. He brought down the Jedi to a small number and, likewise, eliminated all known Sith and the end of the series. And it was all in the parameters character's freewill and choice.
In short, whatever Anakin/Vader felt was the correct path (jedi or sith) would be the path he would fight for throughout the series. You could argue that the dark path was still ultimately destined for him to go to. But barring the original trilogy needing to happen, it wasn't simply because Anakin was created to be evil. Through the series, he was given many choices and options on how to solve his problems (as a Jedi or as a Sith). And it was Palpatine who kept pushing him in a certain direction. And, for a while, Anakin was able to push back some of that temptation, only given in at times that most people would consider more than appropriate (like the death of his mother). And I think it also helped that he had Palpatine and Obi-wan acting as his conscience for a fair portion of the films as well until Palpatine found an opening to turn Anakin into his puppet.
That's my speculation... course I could be way off from what Bob is thinking. I should probably refresh myself on the movies as it's been a couple years since I've seen em and I probably missed a few important details.