I think everyone who is so anxious to believe that Dumbledore never used Dark Magic is forgetting two things:
  1. Alastor "Mad Eye" Moody (the Auror, not the imposter Crouch Jr) was given special dispensation from the Ministry to use the Unforgiveables against the Death Eaters and gives every indication of having done so when he could not at all possibly avoid it (but that he did his level best to avoid it when at all possible)
  2. Killing a person is illegal in the Muggle world, too, but it's accepted as a necessary evil in matters of self, home and homeland defence, and in the case of a police versus criminals - or even SUSPECTS!

Dumbledore was fighting a war against Grindelwald, not having a minor disagreement over the paint colour for the wall of their boudoir.

If, by killing one completely evil person you can save the lives of countless innocents, then you are morally obligated to take that person out of the equation.

In this case, things are not so easily "black and white" - there are tons of shades of grey in there, and you can Levitate someone over the edge of a cliff and then remove the incantation, and when they hit the bottom they are no less dead than if you hit them square in the face with an Avada Kedavra.