Answer:
We have to look at this from the government's point of view. Truman wanted to prevent Japan from participating in the current wars. What he did was for the best of our nation, and most likely others. Japan had the Imperial Japanese Army, with 51 divisions and special purpose artillery, cavalry, anti-aircraft, and armored units with a whopping 1,700,000 people. That was only 1941, by 1945 there were 6 million soldiers in the IJA. They were a force to be feared. And Truman felt threatened and acted out to protect America. So I believe his actions were reasonable, though it resulted in the death of more than 80,000 people. There were rumors that Truman attempted to warn the great cities before they were destroyed.
I really hope this helps you, even if only a little...!