With the express aim of exterminating their populations? I wouldn't go that far. But I think it was a immoral. And they did very little damage to German or Japanese war production as well and in fact- installed an even greater resistance in both populations to fight to the end. It is never remembered- but it was Great Britian which started terror bombing of civilian populations. Great Britain bombed Berlin first and this so infuriated Hitler that he ordered the Blitz. The effect of the Blitz in Great Britain was to harden resolve and it actually boosted morale for the war. Now why would American and British bombing over Japan and Germany not have the same effect on the population? The answer is it did. Both countries fought harder as a result as they felt such an enemy that would bomb civilians is so brutal that they must be fought at all cost. Soldiers coming home in Germany on leave to find their relatives dead were apt to fight harder and they did.
But if you want to get into atrocities- nothing beats the behaviour of the Red Army in every occupied land they conquered. The barbarity is well documented. Even allies of Russia like Tito protested to Stalin as to the behaviour of his troops!
First of all, Britian bombed Berlin 6 times before an adequet response. It was not to harden moral but to take the pressure off of the RAF which was being slowly exterminated by superior numbers and production. When the US entered the war it picked right up. They started with walled cities, it was easier to control the fire that way and get the formula of bombs just right. The formula ran something like this: First came the hard bombs to break up the buildings. Then after enough damage was done, came the waves of phosperous bombs to create fires that could not be put out by water. Following this came a mixture of anti personnel bombs to destroy rescue crews and civilians running for cover.
This bombing continued for days until the flames reached heights of a mile or more and hurricane strength winds snapped trees of 1 meter diameter at city edges. All were killed, either axphixiated or combusted in their bunkers and cellers. Accounts range of civilians trapped in burning asphalt. Extermination was the goal because hardened industry was not targetted by the West, it was targetted by the East, by Russia. Talk to some old Germans, I did, I spoke extensively with many witnesses. I wrote a term paper on the subject. By book references, incidently, were mostly American and British writers.