The deliberate killing of civilians for any reason whatsoever, whether as a means or as an end, is gravely morally wrong.
Did the bombing of Hiroshima and Nagasaki save the lives of thousands of US troops, and thousands of Japanese civilians as well, in the short or long run? We can't be sure of that. Using the atomic bomb to destroy a place that far fewer people but huge psychological value (e.g. the top of Mt. Fuji) might have also saved those lives. Maybe, maybe not. If we'd done it, we'd have soon found out. But the deliberate incineration of a city, together with all its innocents, cannot be justified because God forbids murder.
Furthermore, in the case of the deliberate targeting of a city as such, together with its inhabitants, the resulting deaths cannot be considered "collateral damage." This is because the deaths were not only foreseen, but intentional.
From the point of view of moral law, the deliberate killing of civilians --- whether with bullets, or conventional bombs, or knives, or nukes, or fueled-up jet airliners on a deliberate collision course --- is megabortion.
Both cities were centers for the war industry.
The US was not out to kill civilians. Both cities were legitimate targets. Hiroshima was the home of Headquarters, II Army Corps, the 5th Division and number of other units. It was one of the few cities left for bombing.
Nagaski was naval shipbuilding and repair center and had been bombed before.
Did the dropping of the bombs save lives? The Japanese still had over 2 million men ready and Allied estimates predicted Allied casualties from 200,000 to a million and the war would continue until the end of 1946.
You have the right to your opinion. All I know is that I wouldn't want to be part of the Allied invasion fleet against Japan. And I'm sure many of the people who were slated to go were glad they didn't have to invade.