It is natural that we should be shocked at the random slaughter of inoffensive strangers by the Tokyo knifeman Tomohiro Kato, just as we were by the horrific destruction wrought by the disaffected student Seung Hui Cho at Virginia Technical College last year.

But we should also be troubled by the nature and source of this reaction. We profess to be shocked on behalf of the wretched victims. In fact, our dismay is far more personal and possibly a little less worthy. It is counterfeit for self-protection, a beautifully disguised evasion of a truth known in our soul, but denied by our watchful intelligence.

The truth is this - Kato is aberrant, but not inhuman; for predisposition to violence is an inherently human characteristic that we are reluctant to admit. The purpose of moral codes and norms of behaviour is to restrain and inhibit the natural human urge to lash out, sometimes for no other reason than to satisfy a profound nihilistic fury against the meaninglessness of life.

Kato apparently went out to kill people because he felt like it. In Dostoevsky's Crime and Punishment, Raskolnikov murdered as an experiment, to see what it would be like. In L'Étranger, by Albert Camus, Meursault killed because he was fed up. Meursault is not really a “stranger” or “outsider”. He is like all of us, but mercifully, the rest of us contrive to smother this nihilism and useless violence, lest social existence become impossible.

By expressing shock at people such as Kato, we deflect the more disturbing shock of recognition - that, in the wrong circumstances, each of us could have been him.

A variety of influences might upset the equilibrium that we guard to prevent a lurch into the moral void. It is not necessary to penetrate the lexicon of psychiatric disorders and personality malfunctions, although some may apply in certain cases. One has only to look at history to see that violent and cruel behaviour is the hidden secret of mankind, needing only the sanction of authority to burst into noxious bloom. Nazi Germany is the most available and uncomfortable example. As a boy, I assumed that there were half a dozen concentration camps in the country, where disgusting wickedness was kept apart from the citizenry as a whole. It was a dizzying and ghastly revelation to discover there had been many more, and that thousands of ordinary respectable men had been engaged in the routine humiliation, degradation, torture and murder of fellow human beings.

They had been unremarkable greengrocers and stockbrokers, good husbands and devoted fathers, before the Nazis encouraged them to nurture their latent nihilism. They killed at whim, because the State made it acceptable. It might even have appeared fun at times. Every one of those concentration camp guards could have been Tomohiro Kato the other day; they would not have been shocked by his behaviour, for self-protection was no longer necessary - their moral compass had already been skewed.

The Nazis were not alone and it would be as nasty to suggest that their behaviour was a cultural phenomenon - exclusively German - as it would to imply that the Tokyo knifeman was typically Japanese. This featureless nihilism transcends national frontiers and cultural heritage.

The Spanish Catholic Church was notoriously barbaric, with murder sanctioned this time by God. The British have on occasion descended into atrocious disregard for life, especially if that life belonged to an IRA prisoner or sympathiser.

The Americans have been passing through a profoundly immoral period for the past seven years, treating fellow humans at Guantanamo Bay and Iraq with the same indifferent contempt as any criminal lunatic like Kato. A young American soldier who strafed a British vehicle came so close that one could see a broad grin on his face as he came in for the kill (in “friendly” fire). And it is repellently obvious that the Arabs who attacked the World Trade Centre on September 11 were utterly unconcerned about the human life that they were destroying.

These examples share that element of nihilistic vacuity, of moral absence, which drove a seemingly innocuous Tokyo man to plunge his knife into strangers. Destruction is the key, not with a conscious will to destroy, but with a powerful unconscious need to do something that is noticed.

We are a jumble of needs from the day we are born. The need to be cherished and nurtured; the requirement for love; the need to succeed; the requirement for praise or “respect”; the need for food; the requirement for ambition; the need to soar; the sometimes intoxicating need for adventure.

But if some of these needs, or several at once, are thwarted, there always remains, as a last resort, the need to be effective, to do something with a consequence of sorts.

Destructiveness is undoubtedly effective. It gets attention. Sadism is effective. Fred West derived more satisfaction from the poor girls that he tortured than from his willingly subservient wife.

Perhaps those Nazi greengrocers were more effective doling out life and death than selling lettuce. Whatever the political motives of their masters, perhaps the young men who flew into the twin towers felt that at last they were doing something that mattered.

Perhaps, the present explosion of teenage knife crime in urban centres is relevant. These youngsters have food on the table, money in their pockets. But if they know that they will always be kept down, distrusted, harassed, hunted, they might resort to being effective - to making sure that somebody notices them, even if only another youngster who showed insufficient “respect”.

Tomohiro Kato had no grudge against his victims. His grudge was against the pointlessness of life, of his place and time on Earth. Newspapers have reported that he left messages saying that he felt he had no future, was ugly and “worse than trash”.

The same is true of Seung Hui Cho. The bitter, incoherent explanation that he left behind indicates that he was not angry with school, parents or authority, but with life, with fate itself. Albert Camus can turn such a character into art, but in reality, we cannot spot them, for there is something of them in all of us, lurking, fortunately suffocated and impotent, which we acknowledge at our peril. That is what shocks.

Brian Masters is the author of numerous books, including The Evil that Men Do and Killing for Company

Times Recommends

• In the real world, the public sector must pay
• Playing politics in a land of double Dutch
• Be like the plinth people: not set in stone