MASS-MURDER has nothing to do with biology. The fundamental idea generating mass-murder is not "killing"--but getting people of other societies to die--in the name of one's own society's sacred ideal.

Mass-murder is a societal mechanism orthogonal to the quality of the idea; or what we think of it.

 The Japanese in 1905 were mass-murderers.

The Nations of the First World war were mass-murders (killing in the name of France, Germany, Great Britain and many other countries): nine-million dead.

Lenin was a pioneer, murdering in the name of communism (1919), followed by Stalin, Mao and Pol-Pot.

Harry Truman was a mass-murderer (one-hundred thousand dead, never lost a moment's sleep over it), ending the Second World War, although we Americans understand his murdering as necessary and justified.

Lyndon Johnson was a mass-murderer who sought to destroy communism in Vietnam, followed by Nixon who sought to destroy communism in Cambodia.

These killings was undertaken in the name of SACRED IDEALS  (Scott Atran) that many believed were more important than human lives.

Why do these ideals require mass-murder? We don't actually know, living in the midst of a delusion that we barely comprehend.