The Mongols also destroyed irrigation systems over large areas effectively turning once verdant and productive tracts into wasteland. And that 40 million figure may be way under the actual total slaughtered or killed indirecly as a result of famine and disease that followed in the wake of the very destructive Mongol armies.
I believe we get insight into the kind of thinking that is pervasive among the inner circle of the green movement. Fewer humans = less carbon emissons = happy Gaiea. Not a few of these people would be quite happy to see wholesale slaughter of their fellow human beings. Their biggest worry in regards to that is means and not the end result (which they welcome). So long as people can be exterminated without filling the atmosphere with nuclear fallout or excessive carbon emissions from the burning of whole cities then it is OK (though “regretable”).
This describes Peter Singer. He was quoted in the WSJ very recently in an article discussing the merits of saving human life in poor countries.
His opinion of course, is that life there should not be saved or even brought forth.