I've hesitated to bring this up in this context - after all "two wrongs don't make a right". However:
I think it would have been politically close to impossible for Churchill, Harris, and the other British leaders of the time to say "we're not going to bomb German cities" to a populace that had been living through the Blitz.
It's easy in peacetime to condemn certain wartime actions - it becomes completely different when the bullets are flying. Similarly, had Truman decided against using the A-Bomb against Japan, and an invasion had been necessary resulting in hundreds of thousands of American casualties, the American public would have crucified him (perhaps literally).
Preventing civilian casualties during war is a relatively recent concept - while specifically targeting civilians was uncommon, as a rule little attention was applied to avoiding civilian casualties .
Siege warfare was common place for centuries - and it's success was based on starving the populace - including civilians - of food and water until they were forced to surrender - often accompanied by indiscriminate shelling and the inevitable civilian casualties.
As weapons of war became more deadly, casualties to both combatants and civilians have increased correspondingly. It's only the advent of so called 'smart' weapons since WW II that has allowed the precision to largely avoid harming civilians (and those are still far from perfect).