In France, in Tunisia, in Kuwait – horror upon horror, in a single day. It played out like some kind of gruesome auction, each atrocity bidding against the others for our appalled attention. The opening offer came near Lyon, where a factory was attacked and, more shocking, a severed head was found on top of a gate, and a decapitated body nearby. The French president said the corpse had been inscribed with a message.

From the Tunisian resort of Sousse, holidaymakers tweeted terrified pictures from their barricaded hotel rooms, describing how they had fled from the beach after sounds they had assumed were a daytime fireworks display turned out to be the opening gunshots of a massacre. From Kuwait City, as if to top the rival bids, a suicide bomber walked into a mosque packed with 2,000 people and pressed the button that he hoped would send scores to their deaths.

Each of these acts pulled our gaze from the event its perpetrators had surely hoped would trump all others. On Tuesday an Isis video – “snuff movie” would be the more accurate term – showed five Muslim men, each wearing a Guantánamo-style red jumpsuit, packed into a cage and lowered into a swimming pool. State-of-the-art underwater cameras recorded the men’s dying minutes, the thrashing and flailing as they drowned. (I rely here on reports: my small stance against the so-called Islamic State’s propaganda war is to refuse to watch its propaganda.)

What are we to make of these events? What are we to do with what we have witnessed? Experts will look for connections, for common authorship. There will be claims of responsibility. Islamic State has already sought credit for the deaths in Kuwait. There will be analysis aplenty of IS’s position, of the global response, of the nature of contemporary terrorism.

But a simpler thing connects these horrendous incidents. A clue to it came in a quieter moment, one all but lost in the calamity and grief of this bloody Friday. The Queen visited Bergen-Belsen, a Nazi concentration camp liberated 70 years ago, where unspeakable brutality reigned and where 50,000 lost their lives.

Within months of the war’s end in 1945, the political theorist Hannah Arendt wrote: “The problem of evil will be the fundamental question of postwar intellectual life in Europe.” She meant that after the Holocaust – when Europeans had seen what they were capable of – the dominant concern would be understanding how such horror had been possible. As it happened, that contemplation of the Holocaust did not come straight away, and it did not come everywhere. But it did come.

A battle is under way for civilisation, that should unite the great religions of the world against this tiny death cult

In the process, “the problem of evil” became shorthand for a particular challenge to people of faith: how can one believe in a benevolent, all-powerful God when the world contains such wickedness? The Nazi murder of 6 million Jews, including more than a million children, almost became a point of theology, a standing rebuttal to the possibility of a good God.

But what we saw today confirms that “the problem of evil” is not a historical question. It is a problem of now. And nor does it challenge only religious believers. It surely vexes all of us, as we contemplate a world where such cruelty can happen – whether it was worshippers gunned down in Charleston last week or blown apart in Kuwait today.

I know there are coherent, substantial explanations for all these events. Historical legacies, geopolitical forces, local factors: they are all relevant, but they only go so far. They do not reach the heart of the matter: how is such horror possible?

Take the example of the underwater drowning, so carefully staged and slickly filmed. Anti-Shia sectarianism might explain the killers’ hatred and therefore their motive. The 2003 invasion of Iraq helps explain the killers’ capacity, their ability to wreak such violence: they are only able to rule in Nineveh because US-led forces destroyed the Iraqi state. But that doesn’t explain the sheer sadism on show, the ability of one human being to inflict not just death but such a painful, humiliating death on another.

One option is not to regard it as puzzling at all. Recall the character in Woody Allen’s movie Hannah and Her Sisters, whose response to Arendt’s problem of evil was to say that people who ask how the Holocaust happened are asking the wrong question. “Given what people are, the question is, ‘Why doesn’t it happen more often?’”

The crucial phrase there is “given what people are”. Such a stance rests on a bleak view of human nature. If we believe that people are innately savage creatures who delight in each other’s pain, then it is no surprise the young men of Isis will try to pile one atrocity upon another – torching a man in a cage, executing children, forcing a child to shoot dead an adult – turning sadism into a competitive sport.

But if we have a different view of human beings and their capacity for love and empathy, then the problem of evil persists. We can fall back on psychology, suggesting or even hoping that the men behind today’s horrors were simply unhinged, damaged individuals, no different in kind from Nicholas Salvador, who was committed indefinitely to a secure psychiatric hospital this week, for his beheading of Palmira Silva, an 82-year-old north London grandmother.

Or, if we decide the killers are sane, we can look to the psychology of groups. We can remind ourselves of the post-Holocaust work of Stanley Milgram whose experiments on obedience demonstrated a human willingness to inflict great pain, just so long as one was following the instructions of a trusted authority.

We might turn to those at the cutting edge of “philosophy of mind”, who argue that the self is a collection of fragmented character traits. “In a unified self, it all fits into a coherent whole,” Professor Quassim Cassam of Warwick University explained to me. “But in others, the self is made up of different fragments which are not coherent.” Such people can compartmentalise, complying with human norms in one part of their lives while violating them in another.

A simpler explanation is that the butchers of Islamic State are following an age-old military tactic, one that would have been recognised by Genghis Khan and Attila the Hun: terrify the enemy. Isis drowned those men to make us tremble.

It works too. Holidaymakers will abandon Sousse, at least for a while. But while these crimes sow fear, they also prompt revulsion. And that revulsion is shared. I spoke yesterday with Usama Hasan, an Islamic scholar and one-time jihadist. He spoke of his “disgust” at the evils committed this week, noting how alien they were to Islamic scripture which forbids, for example, the desecration of a corpse.

He said that a battle was under way for civilisation, one that should unite the great societies and religions of the world – Christianity, Islam, Hinduism, Judaism and more – against the vicious death cult that is violent jihadism. It would be a nonsense to speak of such a struggle as a war against evil. A war like that could never be won. Evil is within us and it is, apparently, perennial. But we must not be afraid to name it for what it truly is.