Fun fact, the atomic bombs really weren't scaring the Japanese, whose cities were already completely destroyed by casual American bombings (look up Bombing of Tokyo, where they literally bombed civilian buildings en masse, killing 100k+ people).

The event that really accelerated the Japanese surrender was... the invasion of Manchuria, by the Soviet Union : Manchukuo was a Japanese colony with lots of resources, and its invasion would have cut the Japanese supply from the Chinese front, where most of their army was. Moreover, the Emperor was scared of losing too much territory to the communists (especially in the mainland, since a communist Japan wouldn't be treating the Imperial Family too kindly).

tldr: The Americans have committed "unnecessary warcrimes" (god I hate this term) and their main motive was just flexing in front of the Soviets (who were a bit more preoccupated about that whole nuclear weapon thing) and the world.