The justification of the WWII Allied Strategic Bombing Campaign in Europe is not that the Luftwaffe had flattened our cities (they didn't drop a single bomb on the USA,
AFAIK), but that it helped us to win the war. I strongly believe that was the case.
No-one knows what would have happened if we had not conducted it as we did, but crucial turning points in our favour such as Kirsk and D-Day would most probably not have been possible, leaving Europe in the thrall of the Nazis and the fall of Moscow a matter of time only. Our demise would have not been far behind unless the atomic bombs had been dropped on Germany, as intended. The monster was war, not Harris. If you have to fight a war, fight it to win or just don't bother fighting at all, for you will lose.