What is the "root cause" of man's inhumanity to  man and  animal, which is the subject of this thread?

It is the same as that of the Sustainability Problem (global warming etc): a socio-economic order rooted in and dependent on man's animal nature and behaviour, which it developed (naturally enough, in view of human origins) both to serve and exploit.

To "serve and exploit". That's the key. 

Millions of years of evolution formed and adapted human nature and behaviour to SERVE the individual and his extended family group in the Darwinian struggle for survival and advantage by EXPLOITING the natural environment, which included other animals AND other, rival, groups of humans (by robbing them, trading with them, enslaving them, using them as allies, etc).

With the advent of civilization and the creation of an artificial socio-economic environment things got rather muddled. Leaders were able to profit from appealing to their group's sense of "family loyalty and mutuality", while at the same time exploiting it for their own individual, family, or class advantage. In the middle ages, these developments, directed by an alliance of aristocracy and clergy with a common interest in exploiting the peasant masses, led to the creation of the "nation state".  It was there to serve society as a whole, they claimed, which to some extent it did, but its primarily purpose was as an "instrument of exploitation", in the same way that a cattle farm is, serving the needs of its cattle, in order to exploit them for the benefit of the farmer.

Aristocracy and clergy have largely been displaced by other elites (in politics, business, the media etc.), which are less rigid, more "meritocratic" and theoretically, at least, open to everyone, but the "primary function" of the nation state remains the same: the maintenance of society in order to facilitate its exploitation.

The source of virtually all our social, economic and environmental problems lies in our failure to recognise that the "primary function" of our socio-economic order (the nation state and the economy) is not to serve society, which we rationalize and delude ourselves into believing, but to exploit it.

Why has this profound truth, which follows logically from Darwin's theory of evolution and human origins, not been recognised or acknowledged by those academics whose fields of expertise it falls in (anthropologists, evolutionary biologists, social scientists, psychologists, etc)?  The reason, I think, is because, like everyone else, their total immersion in and dependency on the existing socio-economic order blinds them to it.