I haven’t seen the film yet so I don’t know if they get into this, but a large number of the scientists involved with the Manhattan Project were working because they were terrified that the Nazis would build a bomb before the Allies. When, for several reasons, that failed to happen, they were relieved that the bomb wouldn’t have to be used. They felt betrayed when it was used against Japan, who were not developing a bomb and who could have been defeated using conventional means.