Every so often on historical subreddits I'll see a debate about the morality of using the atomic bomb. I'm sure you've all seen the arguments, so I'm not going to address them. But were there any critics after the bombs were dropped in 1945? Did anyone find them unnecessary or cruel?
It is an interesting question. I've studied around this area (as a Cold War historian) for some time, and though some of the scientists who worked on developing the Atomic Bomb rapidly became critical of the dangerous possibilities for the destruction of humankind that it allowed, it was several years (some time around 1948-9) before the general public really started to have anything like an understanding of the potential of atomic weapons.
Two books upon the moral development of targeting civilians in the Second World War I can't recommend enough are 'Moral Combat' by Michael Burleigh and 'Among the Dead Cities' by A. C. Grayling. Whilst the former discusses the moral devolution towards both sides committing atrocities against each other, the later looks in depth at the 'area bombing' debate. This centres around whether targeting civilians is acceptable even in a total war. Many on the Joint Chiefs of Staff saw that the bomb was the least bad option open to them, with the possibilities being:
Continue LeMay's conventional bombing campaign on smaller and smaller urban targets until the Japanese had no more capacity to resist. This course had already led to the deaths of 30 times more Japanese than the Atom bombs had killed.
Due to Allied control of oil producing areas and the allied naval superiority achieved by 1945, there was due to the limited agricultural production of Japan the possibility of blockading the Home Islands until widespread starvation forced the Japanese government to surrender. This would obviously lead to significant numbers of civilian casualties if it went on for any length of time.
3)a) Allow the Soviet forces to invade the Home Islands, or a commit to a joint invasion by the Western Allies and Soviets, this would lead to a power-sharing agreement with the Soviets, as happened in Germany.
b) A US invasion of the Home Islands (Operation Downfall) that anticipated the Soviet invasion and ensured US domination of Post-war Japan. Both a) and b) would have led to significant numbers of casualties, both of US soldiers and Japanese, as shown by the very high numbers of civilian casualties on Okinawa.
Thus 4) became the use of the Atomic Bomb, which as a new weapon had the possibility of shocking the Japanese into realising they had to surrender. If this failed, then options 1-3 were still available.
Finally to answer the question, some within the scientific establishment who understood the bomb rapidly became critical of its past and potential use, but policy making elites in the military and government thought of it as the least bad option (thought they didn't understand fully about fallout and radiation sickness). Civilians outside of this small elite were generally uninformed of the true nature of the bomb and thus it can not be separated from their general feelings about 'area bombing' and the morality of the war as a whole.