[berntb]

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).

[maxerickson]

It can just be an unfortunate synthesis of advantageous traits.

Detachment is pretty clearly a (potentially) useful trait for a leader. Same thing for stuff like charisma.