Mad Madam Mim

In the animated Disney movie "Sword in the Stone" we are introduced to a villain named "Mad Madam Mim."
In this prequel to Sword in the Stone, I portray a young Morgein leFay, struggling to come to terms with her powers and the fraught political situation she lives in.
Morgein, a natural, meets Merlin, who comes to his magical powers through dispassionate study. Morgein and Merlin find themselves becoming quick allies in trying to use their (officially condemned, covertly accepted) magicks, among other gifts, to effect positive change in their medieval world.
Morgein struggles to keep up appearances for Merlin's high academic and moral standards as the allure of dark and destructive magicks pull her. She can control them. She can use them to unite the kingdom under a new (and pliable) king. Surely Merlin, the starry eyed idealist, will help her achieve this noble aim.

How does she become an unhinged loner out in the woods waiting to destroy Arthur when he happens upon the place?
Why does she seem to be such bitter enemies with Merlin? What happened between them growing up?
If Merlin represents the enlightenment as he seems to in Sword in the Stone, what does Madam Mim represent?
Is the story that people tell, that Merlin placed the sword in the stone really the truth, or is it just a story that the winners have told?