Reading a biography of Oppenheimer explores much of this. One thing is clear, the bomb was used on an essentially defeated enemy. From another angle though, it was necessary to push back Stalin and could be considered not as the last blow of WWII but the first blow in the cold war.

It is difficult to question the possible outcomes if different choices were made but you can be sure that the last 80 years could have been much different and much worse. The bomb did make certain kinds of war impossible, and it is not clear if that would be true if they had not been used when they were.