Civilians did not do that
Ethics does nothing to protect the innocent.
It has not done anything near the magnitude of Imperial Japan or Nazi Germany.
If the United States engaged in _true_, genocide, killed millions, or annexed other countries, then I would fully support a war against it.
I don't believe that the United States has done more wrong than good in its existence purely because of the atrocities of the Axis.
If your government engaged in widespread offensive wars that leave millions dead, then I think that your civilians become a valid military target.
If your country isn't respecting the other civilians, then why would you expect another country to respect your civilians?
At a time when we have seen the Israel-Palestine conflict ratchet up to the worst levels of violence in decades, maybe we should be hesitant to accept this position. This kind of thinking on both sides there is part of how and why they are where they are.
To phrase it a little better: if some country is committing atrocities at scale, then I believe any amount of force is justified to bring that to an end.
As for why, IMO the strong militarism that was part of their culture in that era is probably a large part of it: https://en.wikipedia.org/wiki/Japanese_militarism
That said, there is a difference between you voluntarily signing up to join the military and you being dragged into it.
Maybe not directly.
100s of thousands of people don't commit atrocities just out of the blue.
Their doctrine was pretty hardcore.