Something interesting I heard/read awhile back. Not sure if there is any truth to this, but apparently it was originally planned to reveal he was a pupil of Mama Odie in his younger days. She taught him voodoo to help others for good, as she does. But when he became consumed with powerlust, he turned to his friends on the other side to develop darker magic.
Again, take it all worth a grain of salt, but that would have been a cool reveal within the movie.
Edit: And if there is ever a live-action remake (considering Disney's track record lately, that's already a given).
And if that plot point actually was intended for the original, I hope they'll consider adding it.