Those who have the greatest potential for good also have the greatest potential for evil. Anakin needed to be shown as this pure-hearted, innocent young boy to contrast with who he would become. The tragedy of it all is more pronounced that way.
Satan was the Angel of Light, the most powerful of all the angels and the only being who was considered second to God. Thus, all of that potential for good also created the potential for a massive fall that changed the course of the entire creation for all eternity.
So, I believe Lucas started off on the right foot, for the most part; but lost his way somewhere in the process and didn't realize it until it was too late.