In the book, it had described all the ways magic could become uncontrollable in adults, and she recognized hers had been a result of her soul being fractured by casting the killing curse. It had been a suspicion of hers since she had discovered the problem, but she had been unwilling to accept it until she had thoroughly researched the matter. It pained her to think her soul was no longer as whole as it once had been, but she cleared her mind of that quickly. Every time she looked into her mind, it never ended well – all of the images of Ron and Greyback would dredge up to the forefront and torment her for hours on end.