My canine guest uncovered a nest of shrews in the garden a couple of days ago. He murdered the brood members who failed to escape in under five seconds.

He displayed zero contrition about his act. Two minutes later, offered a pine cone to chase, he’d forgotten all about the multiple murder.

My canine guest is – in purely anthropological terms – a heinous psychopath. Except he’s a dog: uncomplicated and uninfluenced by ideas about the sanctity of life, the morality of vegetarianism, or why milking cows is theft.

Until quite recently – some 20,000 years at most – our lives as hunter gatherers depended largely on having the same, unapologetically murderous instinct. We were –  just a geological blink ago – equally psychopathic.

But there have been two connected dimensions to our development since then. The first is that our larger speculative brains have given us the introspection to wonder about whether killing living things is a good or bad thing – especially each other; the second is that the organisational brain has enabled societies to develop wherein the vast majority of us no longer need to kill in order to eat.

These developments are purely experiential. They are, neither of them, evolutionary. Beneath this anorexically thin layer of moral socialisation, we are still the same killers we were.

The evidence supporting this view is overwhelming. After 1933, the vast majority of the German nation went along with unjustified invasion of its neighbours, and then human extermination on an industrial scale. After 1926, the Russian nation acquiesced in a system of obscenely unjust execution and incarceration that lasted almost thirty years. During the period 1978-97, the Cambodian leader Pol Pot presided over the organised death of some 38% of the population.

Those who choose to ignore or deny this endemic element of our nature are doomed to repeat its worst excesses. Those who choose to exploit it are once more in our midst.

The need for vigilance remains ever-present.

Earlier at The Slog: Why the Scottish Play still haunts British politics