This week’s news of the arrest of Radovan Karadzic, the Bosnian Serb leader who is accused of the genocide of many thousands of Bosnian Muslims, prompted me to reflect on the baleful presence in history of what the philosopher Immanuel Kant called “radical evil.”

What did Kant have in mind? In the absence of the legal or moral constraints of civilization, mankind descends into barbarism with terrifying swiftness. Radical evil is, by definition, ineradicable — it is rooted deep within us, beyond the reach of culture or society.

Radical evil is what Augustine meant by original sin, but the truth that such abstract concepts embody is even older, at least as old as the Hebrew Bible. The third chapter of the Book of Genesis tells the story of the expulsion of Adam and Eve, a story so sublime and awesome that it provided the material for John Milton’s “Paradise Lost.” The fourth chapter of Genesis tells the even more terrible story of Cain and Abel, which ends with the whole of humanity bearing the mark of Cain’s fratricidal crime.

The mark of Cain was all too visible in Bosnia under Mr. Karadzic and his henchman General Mladic — still at large, protected by the Serbian army — as they destroyed the fragile balance between ethnicity and religion which had hitherto preserved the peace between Orthodox Serbs, Catholic Croats, and Muslim Bosnians.

Why does radical evil emerge with such unpredictable ferocity? We see it now in Iraq and Afghanistan, we have seen it across Africa, Asia, and the Middle East. And we saw it reemerge in Europe during the early 1990s, on a scale that we had naively assumed was no longer conceivable after the Holocaust. But for the American intervention in 1995 and again in 1999, it is quite possible that war would still be raging in the Balkans to this day, and that the likes of Karadzic would still be in control of Serbia.

The lesson of Mr. Karadzic is that radical evil must always be kept in check by the moral law that Kant similarly located within our human nature, but which Jews and Christians see as a gift from the God in whose image we are created. Even that cynical old atheist Sigmund Freud acknowledged Moses as mankind’s greatest benefactor. In the tablets of the law, the danger we human beings pose to ourselves encounters an immovable object.

Yet over the past generation, large sections of Western society have embarked on an unprecedented experiment. We have attempted to set at nought the Ten Commandments, the Prophets, and the Psalms, the Sermon on the Mount, and all the rest of the moral insights we have inherited from the ancient people of Israel.

Instead, we have moral relativism. If there is no power higher than ourselves, if custom and tradition have no authority, then the way is clear for every petty tyrant to lord it over us. Mr. Karadzic was a quack psychiatrist who seized his opportunity in the maelstrom of post-communist Yugoslavia, a culture in which moral relativism had held sway for decades.

Relativists are always atheists, but atheists aren’t always relativists. In the 19th century it was possible for a devout Catholic such as Lord Acton to pay tribute to an equally devout atheist such as George Eliot. “She created for herself, in the most unfavourable surroundings, under the darkest auspices, such a high, ideal morality that its general influence is elevating and beneficial, and through it atheism competes with theism in the ennoblement of mankind.”

Could anyone say that today’s militant atheists competed with the rabbis and priests to ennoble mankind? The very concept of “nobility” in a moral sense sounds quaint. We have grown used to suppressing our moral sensibilities in favor of a debased form of the old Benthamite utilitarian calculus: not the “greatest happiness of the greatest number,” but the greatest happiness of Number One — the apotheosis of selfishness.

Mr. Karadzic cuts a ludicrous figure in captivity, and he may even deserve Hannah Arendt’s overused cliché about Adolf Eichmann: the “banality of evil.” Yet however banal evildoers may be, evil itself is never banal — it is the very opposite. The irruption of evil into the lives of human beings is as old as history, but it must never be treated as ordinary. Radical evil must be fought to the death.

That is why the likes of Mr. Karadzic must be pursued to the ends of the earth, just as Simon Wiesenthal and the Mossad pursued fugitive Nazi criminals. Mr. Karadzic may seem harmless — just a bearded old man. So does Abu Hamza, the hook-handed preacher of terrorism who yesterday lost his appeal in London against extradition to America. But these monsters must be brought to justice, whatever the price.

Should Mr. Karadzic be tried in the Hague, where Milosevic waited five years and died before his trial was over? Surely it would make more sense for Bosnians to show the world that they are capable of giving him a fair trial.

The time has come to remind a new generation of Europeans that their leaders were guilty of a terrible abdication of responsibility during the Balkan wars between 1992 and 1995 and later over Kosovo. It all seems a long time ago.

Yet we saw the same abdication a decade later over Iraq — and now again over Iran. President Ahmadinejad speaks openly of wiping out Israel, while his Syrian sidekick Bashar al-Assad warns that humanity itself will rue the day that Iran is attacked.

Our response to their radicalism is enfeebled by our relativism. Moral relativism doesn’t just vanish like a nightmare. No more does radical evil. We cannot escape them, but we do not have to live in their shadow.

Mr. Johnson is the editor of Standpoint.