War is reprehensible as, once you are engaged, the only options left available to you are all terrible.
No one likes to think about civilian cities being wiped out, but that's often a reality of war (Japan certainly had no qualms about committing some of the most heinous war crimes of the 20th century against civilian populations). The obvious hypothetical everyone always asks is just how many people would have been lost in an invasion of the mainland, but even the conservative estimates I've come across always far and away exceed the two atomic bombs.
When you consider the repeated attempts by Japan to unleash plague carrying insects on the Americans (after having tested them thoroughly in China) and that they were still attempting to find a means of delivering them to the U.S. mainland (San Diego specifically) it's pretty clear this was not your average, reasonable nation preparing to surrender... Their policy just prior to the atomic bombs was to make invasion so difficult that a more favorable surrender could be reached. Specifically, they wanted to preserve the honor of the emperor.
https://en.m.wikipedia.org/wiki/Unit_731#Biological_warfare
https://en.m.wikipedia.org/wiki/Operation_Cherry_Blossoms_at_Night
What can you really say? Nuclear bombs are terrible, but what would have followed if they'd not been dropped is every bit as terrible. The problem, of course, is that when the numbers are this high it begins to feel more like a math problem than considering the lives of human individuals, but that's the nature of war and why it is to be avoided.
Look at it this way, with all you know of Japan's activities across the pacific, does a single person out there not believe that Japan would have used nuclear weapons had it been possible? Pearl Harbor would be an irradiated crater to this day.
