I agree that was one motive. I also agree that was a good motive. But a good motive does not make an act of mass murder good. Likewise, a teenage girl having an abortion so she can finish school (a good motive), does not make the act of abortion good.
To use the word in a slightly different sense, the ends are not proportionate, in the examples. In one case, finish school. In the other, preclude the death of several millions of humans.
Any continuance of the war against Japan that was even theoretically possible, say from around Jan 1945, would have been open to precisely the same moral objection that the atomic bombs were (indeed, the fire bombings of Tokyo in Mar-Jun 45 are a parallel case, differing only the the economy). The same categories of individuals would have died, with the addition of a huge number of allied military, under any other scenario. The only difference, with respect to the Japanese dead, would have been the absolute numbers who died (roughly 4-5 million more) and how protracted their deaths would have been.
Keep in mind that I do not try to convince you or any RC not to follow the moral teachings of the RCC, assuming they are rightly understood. But, as stated, for humanitarian reasons I would make certain that such doctrine would never affect national policy. Be parsimonious with death. It’s hard to reverse it. And hard to convince the odd millions or so who would have died had your understanding prevailed, that their deaths were a reasonable price to pay to satisfy your own morality.
To the extent that this becomes a intramural RC thread, I’ll step aside, unless there is some issue of historical fact I can add. What the RCC teaches on such a matter (as on all matters) is of interest, but not definitive, for me.
GKC