> Human society was beyond brutal then.

Was it really? There is very little evidence of Paleolithic warfare (especially evidence that is direct evidence of war as opposed to perhaps being capital punishment or the like). It's not until you get sedentary populations that you find clear evidence of warfare, and even then, there's still a great many settlements in early periods whose layouts suggest that defense was not a concern.

Or, to put more bluntly, the evidence seems to indicate that it's rather late in the development of "civilization" [1] that warfare is invented.

[1] I'm framing this in the unilinear model of anthropology, which I don't believe in, because this is how popular anthropology treats it.