You're quite right, Miss Bones: we cannot see the future; we cannot be certain a living person will not find a way to commit evil again; and indeed we cannot know whether a person to whom we extend mercy might ever perform a good or beneficial act subsequent to ours.

It was entirely unanticipated that Professor Dolohov should ever have been in a position to help Mr Diggory, and even less foreseen that he should have been so inclined. What I mean to say is that my actions did not anticipate any future return on his part, and might very well not have led to any 'good' of that sort.

I do maintain, however, that while mercy may yield no measurable good, the refusal of mercy does cause certain harm, and not only to the person to whom mercy was denied. Refusing mercy hardens the heart and nerves of the person who, wielding that power of life and death, chooses to extinguish life. And it hardens the sensibilities of those who carry out or bear witness to such brutality.

Allow me to put this to you in another way. When I think of Voldemort, our so-called 'lord protector', I think that what distinguishes him and his actions and his accomplishments is not his mastery of magic (though he is, undoubtedly, one of the most powerful sorcerers in the long history of magic). What distinguishes 'lord' Voldemort is his cruelty and his murderousness, his inclination to kill any who stand in his way or threaten his supremacy.

Each time we make a choice to take life when it could be preserved (however inconveniently), we narrow the distance between ourselves and Him.