Tokyo was already firebombed and terror bombing of civilian cities was a tactic used by all sides in WW2. If bombing civilians is a war crime, then everyone in WW2 is a war criminal. As for the civilian getting bombed, whether the fire melting their flesh away has a hint of radioactivity is a really minor addition.

That kind of reasoning can be used to justify all kind of things that are considered crimes. Wipe out 200K in one action just to show power seems too much. I don't accept that the amount of creativity used to create weapons cannot be applied also to use them more wisely in ways that don't wipe out 200k.