You have to consider that WW2 was fought before the introduction of the United Nations and the universal laws decreed by the agreement of all its member states. One of those laws is that war is illegal. This is fact- war is illegal, ergo , all people killed in war are killed illegally- and illegal killing can be considered as murder almost everywhere.
So no- the soldiers of WW2 were not murderers according to universal laws that did not exist at the time- although the war itself was in contravention of the Kellog/Briand Peace Pact of 1928. - officially the General Treaty for Renunciation of War as an Instrument of National Policy
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Also- ' Thou shalt not kill ' ain't a bad commandment- even for the ungodly.
Amen.