To make this argument, we should focus on the number of lives that were saved by this decision. It is of course true that the people of Hiroshima and Nagasaki suffered terribly because of the bombs. But it is likely that many more people would have suffered and died had the bomb not been used. The military estimated that 1 million Americans would be killed or wounded in an invasion of Japan. It is certain that Japanese casualties would have been even higher. Therefore, the use of the bombs can be justified as a way of saving lives, both American and Japanese.
The bombing of Hiroshima and Nagasaki gave the Japanese an excuse for surrendering unconditionally. Otherwise, they seemed prepared to fight for every inch of their homeland. Furthermore, once the United States had the atomic bomb it had to show that it was willing to use it. And after dropping the bomb on Hiroshima, it was necessary to drop another one in order to prove that the U.S. had a supply of such bombs. The U.S. wanted to prove this not only to Japan but to the world at large, and especially to the U.S.S.R. It is quite true that the atom bombs actually saved lives on both sides. The Americans could see what resistance the Japanese were capable of by observing what happened at Iwo Jima, Okinawa, and the suicide attacks on the fleet. Most Americans did not have the slightest idea that such things as atom bombs existed or were possible to make. It was all very much top secret.