Japan was the one who started the war. Japan was the aggressor. Defending yourself by winning a war against an aggressor is not a "wrong".
Many more people were saved by dropping the bombs than people died in it. And, while it is a tragedy that those people died, that fault lies with Japan, not the US.
The point of the concept of war crimes is that it applies to you regardless of whether you think you’re in the right. Even if the other side is committing them, it still applies. That’s why the definitions take pains to not make them a winning strategy or unilaterally avoiding them a losing one: why we talk about (deliberately) targeting civilians, not (incidentally) killing them; why putting munitions in a Red Cross-marked hospital annuls all protections for it; etc.
Many more people were saved by dropping the bombs than people died in it. And, while it is a tragedy that those people died, that fault lies with Japan, not the US.