In some Japanese anime one of the characters presented the following paradox:
“What do you do when there is an evil you cannot defeat by just means? Do you stain your hands with evil to destroy evil? Or do you remain steadfastly just and righteous even if it means surrendering to evil?”
Another character denounced the paradox involved when she said "in any case, evil remains." I suppose that paradox is probably famous (or refers/alude to a famous one), but googling about it I can't find its real author. Someone has a clue about it?