While I agree with your general sentiment, here is an anectode that might be considered as evolutionary "softening". Assyrian empire was extremely brutal to pe

While I agree with your general sentiment, here is an anectode that might be considered as evolutionary "softening". Assyrian empire was extremely brutal to people it conquered, Persians who took its place was more "humane" to their conquered subjects as a policy. Resulting in a relatively more stable land and subjects were submitting "willingly". From that point on, it was their main selling point.