The complete argument runs as follows:
Even if one could accept that war goals justify any loss of life, bombings of Tokyo and Dresden type would not be acceptable, because they did not serve those goals at all. Hiroshima might qualify as a demonstration of new technology, but that could have been had equally convincing with much less damage.
As for the real goals, we can only speculate. Revenge must have been among them. I personally think that military leaders - and politicians in their vicinity - inevitably have a tendency to fuel and prolong conflicts. Military engineers and scientist may also enjoy their roles. Barbaric, indeed, and not just all-out (defensive) war.