"The unfortunate fact is that killing has proved to be an effective solution to an array of adaptive problems
in the ruthless evolutionary games of survival and reproductive competition:
Preventing injury, rape, or death; protecting one's children; eliminating a crucial antagonist;
acquiring a rival's resources; securing sexual access to a competitor's mate;
preventing an interloper from appropriating one's own mate;
and protecting vital resources needed for reproduction....
Genghis Khan (1167-1227): "The greatest pleasure is to vanquish your enemies,
to chase them before you, to rob them of their wealth, to see their near and dear bathed in tears,
to ride their horses and sleep on the bellies of their wives and daughters."
We can be sure that the families of the victims of Genghis Khan saw him as evil.
We can be just as sure that his many sons, whose harems he filled with women
of the conquered groups, saw him as a venerated benefactor.
In modern times, we react with horror at Mr. Khan describing the deep psychological satisfaction
he gained from inflicting fitness costs on victims while purloining fitness fruits for himself.
But it is sobering to realize that perhaps half a percent of the world's population
today are descendants of Genghis Khan",
'dangerous idea', http://www.edge.org/q2006/q06_12.html