Many historians claim that the offensive was immoral and unjustified. How can it be right, they argue, for the Allies to have deliberately targeted German cities causing the death of hundreds of thousands of civilians? Even on a strategic level the offensive failed to bring about the collapse of civilian morale that was its intention. Others, however, maintain that the attacks made a decisive contribution to the Allied victory. Vast numbers of German soldiers and planes were diverted from the eastern and western fronts, while Allied bombing attacks virtually destroyed the German air force, clearing the way for the invasion of the continent.
Well, those are the two arguments in a nutshell. What exactly are you asking?
It's important to note that there are two types of bombing during the war: area bombing and precision bombing. Precision bombing was nothing like the precision bombing today, and there was usually quite a bit of collateral damage, but the difference lay in the objective. Area bombing, as a doctrine, was explicitly meant to win the war by the disruption of German morale, which would then have knock-on effects on production and war-weariness. Again, the primary target of area bombing was people and their houses. Precision bombing, on the other hand, aimed to come around to the same end by destroying explicit and specific targets to cause a collapse in production.
This split isn't the same split as day or night, or RAF/USAAF. The British did quite a bit of precision bombing late in the war, just as the USAAF did quite a bit of area bombing late in the war. Anyways, it's important to note that there are these two philosophies in the bombing campaign, and all bombing is not the same. Both claim to save lives in the long run by shortening the war, and, to go into my personal opinion, are both moral things to do in a very nasty and total war provided that they have the effects they claim to have. This is where there is room for debate, and in my opinion, any talk about the morality of either bombing strategy comes down to how effective and necessary they were in ending the war early. And of course, sources and historians differ.
The US Strategic Bombing Survey, conducted immediately after the war by the US air force, concludes that area bombing was ineffective at lowering morale catastrophically, that production at places like Hamburg largely recovered after catastrophic area attacks, and that any dissatisfaction was inexpressible in a police state. Precision bombing, it concludes, was only effective after the correct targets, namely the rail and canal systems, were repeatedly targeted late in the war. It's available online to read for free with a quick google search. Common points to counter this is by February/March 1945 when German production began to collapse, large parts of German territory were already under Allied control.
The Collapse of the German War Economy by Alfred Mierzejewski maintains that the collapse was a direct result of Allied air attacks on the vital coal infrastructure (railways and canals), and goes over why area raids did not work as well. I would recommend that you read it if you are interested.