The US's choice was absolutely correct at the time.
No atomic bombs, no ending the war so soon.
If the Japanese had it first, they would also absolutely have used it on the allies with no hesitation.
What the victims and their families in Hiroshima and Nagasaki want the US and the world to know is, I guess, that their physical and spiritual agony which has been brought by the radioactivity still continues and will continue for more several decades till all of them die.
The number of victims by the bombing of Tokyo on March 10, 1945 was also enormous but the survivors had no chance to suffer such agonies fortunately.
Killing or hurting people by guns or swords manually on the ground is visible and it is easy for both an assailant and a victim to understood how it is tragic but, in case of bombing from the sky, it is not.
An assailant cannot see what has happened on the ground and he may have little consciousness of sin.
He only killed people statistically with his hands kept clean.