The atrocities that the Japanese military committed, consistently and repeatedly and combined with their attack on pearl harbor, meant that Americans of the time on the front lines had few qualms or issues with dropping the bomb.
Truman famously called Oppenheimer a "cry baby" when Oppenheimer expressed doubt. Truman had spent the past decade dealing with the war in his capacity as senator and vice president - seeing the effects it was having first (or second) hand.
Now, this isn't to say that America was right or not right to drop one or both bombs. It may "feel" like I'm saying "the Japanese deserved it for their behavior" - that's not a belief I hold, either.
I just want to provide color for why American leadership seemed so relatively unconcerned with the lives of Japanese civilians in 1945.
Truman famously called Oppenheimer a "cry baby" when Oppenheimer expressed doubt. Truman had spent the past decade dealing with the war in his capacity as senator and vice president - seeing the effects it was having first (or second) hand.
Now, this isn't to say that America was right or not right to drop one or both bombs. It may "feel" like I'm saying "the Japanese deserved it for their behavior" - that's not a belief I hold, either.
I just want to provide color for why American leadership seemed so relatively unconcerned with the lives of Japanese civilians in 1945.