The point was, with the attitude to strangers in clan warfare (not modern post-clan societies), psychopaths have nothing to add regarding ruthlessness. See old Scandinavia.

For stylized cattle raids (e.g. historical Ireland, before the vikings) among old neighbors, there will of course be agreed levels below extermination (or the neighbours will be gone long before western contact).

Cold blooded killers aren't synonymous with psychopaths. It turns out that many of the Nazis weren't psychopaths, but rather psychologically unremarkable people whose institutions and culture allowed them to collectively perform acts of evil over and above what any of them could have done individually. This is called the "banality of evil", and it became widely understood after the Milgram experiment and the Eichmann trial.

Groups of people have always been more ruthless than individuals. Psychopaths are remarkable not in their ruthlessness but in their ability to achieve it all by themselves.

This is a point that the late Jungian psychologist James Hillman raised (and I encountered for the first time) in his book a decade ago titled "A Terrible Love of War." http://www.amazon.com/Terrible-Love-War-James-Hillman/dp/014...

It's different than the normal HN fare, but worth a read. We may be a society of killer apes, but we're not as eager to kill in warfare as you might think. Especially when the other guy was compelled at gunpoint by his government to join the military (that is, drafted) just like you were.