The killing of the innocent ought to be avoided when possible. This applies equally to a mother and daughter huddling in the basement of their home as it does to a conscript father and son huddling in a bunker on the frontline. Deliberate killing can only be justified on the grounds of retribution or necessity.
Retribution
You will remember how the German propaganda films, seeking to terrorise neutral countries and glorying in devastating violence, were wont to show rows of great German bombers being loaded up with bombs, then flying in the air in battle array, then casting down showers of bombs upon the defenseless towns and villages below, choking them in smoke and flame. All this was represented from the beginning of the war to neutral countries as the German way of making war. All this was intended to make the world believe that resistance to the German will was impossible, and that subjugation and slavery were the safest and easiest road. Those days are gone. Though the mills of God grind lowly, yet they grind exceedingly small. And for my part, I hail it as an example of sublime and poetic justice that those who have loosed these horrors upon mankind will now in their homes and persons feel the shattering strokes of just retribution.
—Winston Churchill. Broadcast, London, May 10, 1942.
During the Blitz, the Germans did their utmost to kill as many British civilians as they possibly could. It was only justice for the British to repay the Germans in kind. The only ethical question to be considered was whether the correct degree of proportionality was observed. The bombings of Dresden, Cologne, Hamburg, Berlin, Tokyo, Hiroshima, and Nagasaki unquestionably caused far fewer casualties than those inflicted by the Germans and Japanese, so the answer is undoubtedly yes.
Necessity
I am one of those too who, rather than submit to the right of legislating for us assumed by the British parliament, and which late experience has shewn they will so cruelly exercise, would lend my hand to sink the whole island in the ocean.
—Thomas Jefferson. Letter to John Randolph, Monticello, August 25, 1775.
A people, however small, are entitled to consider their own existence and liberty as more important than the lives of their would-be oppressors or destroyers—however numerous they are. Accordingly, when survival is at stake, proportionality is not required and a nation of 5 million can slaughter 500 million if that is what is needed to defend or avenge itself.
