I strongly dispute the assertion that this is leftist conclusion. Leftists justify the killing of the innocent, or shall we say noncombatants, in theory and in practice, all the time: they kill civilians, captives, class enemies; they deny there is any objective moral law which would classify this as murder; they assert that anything can be justified by consequences, especially (in their "long" view) "progress," a "better human future," and "the judgment of History."
As for the distinction between innocent and guilty being a "European" thing, I will accept that label, if by that we mean these distinctions are at the heart of the moral tradition of the Christian West.
I think maybe you have conflated the collateral deaths of civilians (which are foreseen but not intended), with the deliberate targeting of civilians. These two things are not the same and not even similar. It is a point very important in the thinking of Western Civilization; one which the U.S. affirmed in the U.S. Army Field Manual, and one which the Allies insisted upon at, for instance, Nuremberg.
It is true that the Axis power initiated the policy of total war involving the murder of civilians. That is one of the main reasons why we say the Axis powers were morally depraved.
It is hard to see what "human liberty" means if innocent persons have no right to life. The whole moral justification of a good soldier (I'm thinking of my son, who served in Iraq) and their leaders, is that their intention is to shield the innocent, and smash the aggressor.
I don't believe the record supports this view. Hiroshima and Nagasaki both contained military assets, which is why they were targeted, after a lengthy debate in which the extent of civilian casualties was a major consideration. If civilian deaths was the goal, the bombs would have been dropped on Tokyo, Osaka, or Kyoto, all with greater civilian populations. Kyoto, in particular, was not targeted because of humanitarian considerations.
Furthermore, I don't believe that heads of state can commit the crime of murder against the civilians of another nation with whom they are at war. Murder is a crime that exists only within the jurisdiction of the laws of a state, not between states.
Your views are those primarily adopted by 20th century pacifists, although you don't seem to be one yourself. They are not supported by the “laws of war,” such as they are, as they have been expounded since the 17th century.