The old wizard looked back up at Harry, and said, in a hoarse voice, “He is not like Grindelwald, Harry. There is nothing human left in him. Him you must destroy. You must not hesitate, when the time comes. To him alone, of all the creatures in this world, you must show no mercy; and when you are done you must forget it, forget that you ever did such a thing, and go back to living. Save your fury for that, and that alone.”

We've seen that Dumbledore has worked very hard all his life to be wise and compassionate and generally fit the archetype of the true hero. I can't help wondering if Voldemort is the one person he allows himself to truly hate, which is why that hatred is unusually intense and fully brings out Dumbledore's otherwise unseen dark side (or as fully as Dumbledore has one, which is still less intense than most people's when faced with someone who's hurt them that much).