I mean, sure it's horrible, but again, understanding the context behind decisions is important to getting a full idea of why something was done.
Take something like strategic bombing, which killed more people by a country mile than the atomic bombings. Does anyone bitch on the same level about how many people were killed by regular bombing? Hell, Operation Meetinghouse (the firebombing of Toyko in March 1945) killed something like 150k people in a single raid, and nobody says a goddamned word about it outside of historical circles.
At the end of the day, the idea behind strategic bombing (in the case of the Allies) was that it was a good way to damage the enemy's war effort. The killing of civilians wasn't the objective (unlike the Germans, who explicitly employed terror bombing of civilians as a tactic). Its the cold calculus of fighting a modern war - the enemy's capacity to fight is the ability for them to make more things to fight with, so eliminating that capacity by demolishing factories and houses is a good strategy. The killing of civilians wasn't the objective necessarily - breaking the apparatus they participated in was.
In some ways it's actually better to simply leave millions homeless instead of killing them, as the enemy must house and feed these people instead of using those resources for fighting...
Either way, would you have rather the US blockaded Japan to death to force a surrender? Killing untold numbers of civilians from starvation and disease than a relatively small number of civilians in 2 places? Maybe we wouldn't have needed to if the Russian invasion was enough to scare them into surrender, but we'll never know that for sure...
What would you have done against an enemy that gave every indication they were planning to fight to the death?
Interesting fact about this document is that from what I recall, the air force pushed hard on the idea that bombing alone would be sufficient to win in an effort to secure funding when the US military downsized post-war. I'd fake its findings with at least a little grain of salt.
Also, it's not like we could really have simply sat on our hands until December...the American public wanted results and the cost if the war was astronomical already, so adding on months of mobilization and war economy to "save the lives of a few Japs" (to use the relatively widely held stance of Americans at the time) was never going to happen. To say nothing of the toll on human lives regular strategic bombing and famine conditions would inflict...