The most important thing about Hiroshima was that it was unnecessary, it was at a time when the Japanese were trying to find a way out of the war, and we knew that. However, once it became clear that there wasn't going to be a Nazi bomb, a very different question opened up because then they were building a weapon of mass murder that would have implications for the rest of the planet. The most difficult problem about moral culpability is that this has been lingering more than 50 years and most people have not felt comfortable talking about it. They have been complicit in silence. It also leads to the support for nuclear weapons in general by allowing the moral legitimacy of these weapons to go unchallenged.
Nuclear weapons have questionable use now, and they are becoming a threat to our own security.
The evidence is not conclusive on any of this, because there are gaps in the record. The decisions were made by Truman and Byrnes privately over drinks, during breakfast, at lunch or dinner, or in the back seat of a car on the way to a conference. What you can say accurately as a historian is that the preponderance of evidence points in this direction. But we do not have, yet, all the evidence we need, although every year we seem to find something new. You can begin to eliminate options, and as the picture gets clearer this looks to be the central motive.