So because "the Germans and Japanese routinely bombed cities without the slightest concern for civilian casualties", that made it okay for us to do the same to them?

For instance, if a serial killer slaughters someone's family, does that mean that the law should go slaughter his own family in turn?

If our military inadvertently kills civilians as part of an attack on a strategic target, that's collateral damage and an almost unavoidable part of modern war. However, if our military deliberately targets civilians - as LeMay did - that's murder.

That, sir, is the difference.