If anyone says that the bomb shouldn't have been used then they should go talk to one of the soldiers, sailors or airmen who were to be part of the invasion force of Japan.
My dad for one. 19yrs old.
My Dad was a Marine on a troop ship to Japan to be part of the invasion force when the bombs were dropped. Instead of invading, he was part of the occupation force. He brought home a silk parachute from Japan that was made into my Mom's wedding dress in 1947.
If it hadn't been for those bombs, I wouldn't be typing this right now!
May he Rest In Peace.
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.