The collectivist nationality of Japan (and many other Asian and African nations) would have meant fighting for every inch of every island in the American's push to topple the Japanese government. I would estimate based on my limited research and what I have heard that dropping the bomb on those two Japanese cities probably resulted in fewer deaths that would have the continued traditional bombing runs and an amphibious assault of the main islands. Still, the United States needed a chance to capitalize on its global power projection and the atomic bomb was that.
The ethics should hardly be considered because in order to even get to the point of the bomb you must first justify war from the beginning, which from a purely moral standpoint is hard to do. All of WW2 is not an ethics study but rather one in foreign policy, ideology, and psychology of the masses around the world. Aggression, interventionism, isolationism, or pacifism? Whims of the people that comes with democracy, direct and simple leadership of a small group, or some combination thereof? Will the world respond in fear, resistance, or jealousy to the unleashing of the greatest weapon in humankind's history?
The ethics surrounding WW2 are terrible; every side of every front acted as monsters at one time or another, even the blessed and divine Anglo-American forces. What?! You mean the Brits and Yanks aren't the undisputed "good guys"?! Yes. The war and culture surrounding it were disastrous dark times in the story of the modern world but that isn't what the war was about. It was the retaliation of failing democratic societies to reassert dominance and unite their own people.
So back to the bomb. Justified or Not? It's irrelevant in my opinion. It was war and however terrible the death toll was or could have been, War is a terrible stain on human nature wherever more than 10 humans are gathered under one banner with differing opinions from those around them. You can understand war but it's almost impossible to justify any part in it, including the bombs dropped on Japan.
