"Harry's been bitten, and the resulting cursed wound has my sister's signature all over it. Any attempts to magically heal it result in the wound counteracting or attacking the patient even more. At the end of the Final Battle, I had to do a lot of field amputations with Dark Magic in order to save people whose curses were going to spread and kill them within days. I don't want to take his wand-arm, and even if I do save his life, he might become something undead."