When killing humans became murder

We understand ourselves as human insofar as we define the spiritual forces that govern us as covenantal. By covenantal we mean that the human also has a say in the shaping of the human environment. Peoples who are too close to, and too completely shaped by, the 'natural' environment are experienced by 'advanced' civilizations as sub-human. Sub-humans are classes of human where the killing of them is not considered murder. 

The shift from sub-human to fully human is what happened after Noah's ark preserved man and animal from the Flood. That's when killing humans became murder, while killing animals became a consecrated act. The great catastrophe that was the Flood ushered in a new era of the consciousness of the divine, which consciousness differed fundamentally from the sort of consciousness that ruled prior to the catastrophe. 

"Mankind remembers the history of peoples but has only legendary knowledge of prehistoric tribes. The word 'race' has a precise meaning only when and where peoples are confronted with such tribes of which they have no historical record and which do not know any history of their own. Whether these represent 'prehistoric man,' the accidentally surviving specimens of the first forms of human life on earth, or whether they are the 'posthistoric' survivors of some unknown disaster which ended a civilization we do not know. They certainly appeared rather like the survivors of one great catastrophe which might have been followed by smaller disasters until catastrophic monotony seemed to be a natural condition of human life. At any rate, races in this sense were found only in regions where nature was particularly hostile. What made them different from other human beings was not at all the color of their skin but the fact that they behaved like a part of nature, that they treated nature as their undisputed master, that they had not created a human world, a human reality, and that therefore nature had remained, in all its majesty, the only overwhelming reality – compared to which they appeared to be phantoms, unreal and ghostlike. They were, as it were, 'natural' human beings who lacked the specifically human character, the specifically human reality, so that when European men massacred them they somehow were not aware that they had committed murder." (The Origins of Totalitarianism, Hannah Arendt, 1994; page 192).