Complex sentence: Edmund is repentant and grateful, having come to regret joining the Witch's cause; by the time she had turned a group of animals into stone for telling her that Father Christmas had come to Narnia, he had realized just how evil she was.
Split into simpler sentences: Edmund is repentant and grateful, having come to regret joining the Witch's cause. By the time she turned a group of animals into stone for telling her that Father Christmas had come to Narnia, he realized just how evil she was.