I think Truman may not have had a choice. I was talking to a friend last week that is designing a display for a new museum near K-25,(most of K-25 is being demolished) the building at Oak Ridge, Tn. where the Manhattan Project was going on. He said they had 75,000 employees working on the Bomb in WWII. He said only two people knew exactly how many people were working on the project.
Employees were not allowed to drive into Knoxville or describe anything whatsoever about their job. THey didn't even know what it was, just that they were refining some very high metal of some sort.
My point is that supposedly Truman, wasn't briefed on the Manhattan Project until after Roosevelt died. So there was a huge force in motion, to develop and presumably use this technology as fast as available.
Up until the end of WWII, in the U.S., the military ran wars, and politicians ran the country. So did the military make the decision for him?
Another thought; you all have probably seen the color wartime footage of the Okinawa campaign, where a woman wearing white throws her baby off a cliff and then jumps to her death? Well, the Japanese were telling their people that the Americans would murder, rape, mutilate, or even eat them if captured. SO I think there would have been some unbelievable horrors committed by the Japanese (en masse suicides etc.) rather than be captured.
If the bomb had been ready a couple of years earlier it might have been used on the island campaigns? If the Germans had gotten it is say 1943, they might have won the war.

CHilling thought.