Nuclear weapons do kill enemy people and destroy the enemy's things better than anything else in man's arsenal after all that is the objective to winning any war.
We now have nuclear weapons many times more powerful I hope that it will never be necessary to use them again. We all know how the fourth world war will be fought don't we. Lets hope that as a species were not consigned to be victims of our success in this area.
As for a war crime ? All war is a crime of sorts. I think given the same scenario the US would be justified in doing it again. A true war crime would have been dropping atomic weapons unprovoked.