In war you generally have a series of bad choices because people will die due to them. I take no pride that World War II saw massive civilian death, but I understand how a conventional invasion could have lasted years and cost several times the same number of lives. I don't envy those that made the choice.
World War II had many atrocities, the Japanese weren't immune to committing them as many Chinese will tell you. We can bear old grudges for what happen or elect to move on, which I'd argue has been good for Japan and the United States.
Bombing a city either way as you point out leads to death on a massive scale. Coventry and Dresden being famous examples. If you want to point to a good consequence of Hiroshima is that the Soviet Union and United States seeing the awesome destructive power realized that another war would be mutual annihilation. Since World War II we have witnessed a scaling back of major nation states engaging in war being replaced by commerce. Shame we had to learn such a brutal lesson through millions of deaths including whole cities wiped out in a day, but seems only thing prevent us from engaging in such madness is to raise the stakes that high. At least, for 70 years we have.
