A non-combatant in a country you are at war with is not an enemy, and deliberately killing them is murder. Go take a look at the massacres of Vietnam and how those were treated at the time and as history.
Though, bombing a tank factory and killing a non-combatant janitor is not murder, as you targeted a military target and the death is collateral damage, not a murder. Dresden and the Tokyo bombings killed lots of civilians. Hiroshima was a military target, killing 20,000 Japanese troops. That there were 2 times that in dead civilians doesn't make it murder.
But Nagasaki was a terrorist target. It was smaller, and had only a token military presence. The targets were "civilian" facilities that were making war implements. The factory that made the primary torpedo used in Pearl Harbor was destroyed, as were other factories and metal works. But almost no military were killed. The point was to terrorize the Japanese into surrender. Hiroshima "should have" caused a surrender, which was given with unconscionable conditions. It was a mass killing of the main Japanese force assembled to repel an American invasion from the south. But the Japanese wouldn't give in. So Nagasaki was proof that any place left so far (mostly) untouched would be reduced to a parking lot before an American invasion would lose American lives fighting the Japanese on home soil. It may have been necessary to end the war and killed far fewer Japanese civilians than an invasion, but it was still not a military target.