Military assets can be directly targeted: hopefully, they can be absolutely obliterated. And that's a pretty broad category, and includes all their logistical areas, the storage, armaments depots, supply, port and transshipment areas, for instance, which would include a big chunk of Hiroshima. And noncombatants deaths which are really collateral -- which happen en passant in the course of taking out militiary targets -- are not considered murder because they are not directly intended. (By definition: that's what "collateral" means.)
It makes a difference because we are humans, and what we honestly "intend" makes all the difference in the world. And in the world to come. People who directly intend to kill noncombatants--- the mother who is mothering, the farmer who is farming, kids who are just kidding around --- are in the same moral category as Anders Breivik.
The distinction is that the A-bomb was developed to be indiscrimiante. It didn't just "happen" to be. The strategic intention was to kill a whole lotta civilians in a way that would be unprecedented and stunning. And then the psycholgical and political shock of it would force the war criminals at the head of the Japanese government to unconditionally surrender.
I think (if I'm understanding him correctly) Mr. Hasegawa's point is that the war criminals at the head of the Japanese government were so depraved, that they actually weren't forced by any concern over massive civilian casualties. They had already lost many hundreds of thousands of civilians in the devastation of Tokyo and dozens of other major cities. The Japanese leadership was brutally willing to sacrifice literally millions more, if ---as they thought --- they could drag things out to the point that the US would accept a negotiated settlement.
But when the USSR entered the war (because of the bombings?) the Japanese leadership realized that "dragging things out" was no longer an option. It was going to have to be unconditional surrender, and they'd rather surrender to the USA than to the USSR, because the USA would treat them more decently.
So Mr. Hasegawa's account (again, as I understand it) is really damning of the Japanese leaderhip who were willing to plunge their own people into a flaming abyss fighting island-by-island and city-by-city rather than surrender; and they capitulated only when they realized that the only alternative was dealing with the USSR, which would have consequences far exceeding what they would face with the USA.
At least, that's my short-version take on it.
The Japanese elites were unconcerned about casualties among the common people (as is common with elites around the world).
If the Soviets conquered Japan, the Russians would exterminate the old elites entirely. The secret police would track down the Japanese elites, kill them and their entire families, like they did with the old Russian nobility and the White Russians. They would need to do this in order to re-form the society along their own lines. Machiavelli noted that authoritarian societies were hard to subvert, but easy to hold once conquered, PROVIDED you kill everybody from the old ruling families.
The historical definition of murder, as far as the State and social order was concerned, was the unlawful killing of members of your own nation.
Hence while in Deuteronomy 5 we have the Ten Commandments which include "[17] Thou shalt not kill", in the same book, in Deuteronomy 20 we have instruction on how to deal with the inhabitants of the Promised Land:
[16] But of the cities of these people, which the LORD thy God doth give thee for an inheritance, thou shalt save alive nothing that breatheth:The people of those nations whose customs, practices, and beliefs could not be permitted to contaminate Israelite culture, were to be destroyed completely.
[17] But thou shalt utterly destroy them; namely, the Hittites, and the Amorites, the Canaanites, and the Perizzites, the Hivites, and the Jebusites; as the LORD thy God hath commanded thee:
[18] That they teach you not to do after all their abominations, which they have done unto their gods; so should ye sin against the LORD your God.
[19] When thou shalt besiege a city a long time, in making war against it to take it, thou shalt not destroy the trees thereof by forcing an axe against them: for thou mayest eat of them, and thou shalt not cut them down (for the tree of the field is man's life) to employ them in the siege:
[20] Only the trees which thou knowest that they be not trees for meat, thou shalt destroy and cut them down; and thou shalt build bulwarks against the city that maketh war with thee, until it be subdued.
In Rome, murdering a fellow citizen was a serious offense, yet they had no problem killing the people of Carthage.
In the United States, we killed the Indians until their population was down to insignificant levels. If we hadn't, we would have had our own "Palestinian" issues.
My point is that, in order to pacify a situation, sometimes there is no alternative but to inflict such damage on an enemy society that they lose all interest in continuing the fight. Notice that WW-I had casualties being predominantly among men in the military, and Germany was back for a rematch in just 20 years; in WW-II we devastated cities and populations to the point where the German and Japanese people have lost all interest in war, which probably saved many lives in the long term.
Yours was a long reply, I'm going to answer in pieces as I have time.