The typical argument for this point of view proceeds on two tracks.
First, it is said that the atomic bomb was qualitatively different from conventional bombs. This point of view holds that the power of the single atomic bomb, along with the horrors of radiation sickness, make atomic bombs so different that they should never be used.
Second, there are those who say that there were less terrible ways to end the war. They say that the US could have blockaded the Japanese home islands. This could have starved Japan into submission. They say that the US could have demonstrated the atomic bomb in some way that would make its power clear without killing huge numbers of people. To this way of thinking, there were ways that the war could have been ended without an invasion of Japan and without the use of atomic bombs.