Why did the Woolwich killing happen? Less than a week on, the debate has swiftly moved on to the issue of “preventative measures”, with Theresa May proposing new internet controls and the banning of groups preaching hate.

Yet anyone who dares to use the words “western foreign policy” in this context is bound to be speedily shut up by the likes of Paxman and co. This isn’t because they have never heard of drones and Guantánamo. They are surely aware of the countless thousands of innocent civilians dispatched to their graves by western operations in the Arab world, for whom there are no floral tributes piled on the London pavements. It is rather because they imagine, in their muddled way, that to explain an event is to excuse it. Those who point to the dead of Iraq and Afghanistan are surely doing so as a devious way of justifying the slaughter of a young soldier outside his barracks.

Do they also think this about the crimes of Hitler or Stalin? Are they really suggesting that historians who delve into the origins of fascism are secret Nazi sympathisers, or that to lay bare the causes of the Gulag is to exonerate its architects? The problem for these commentators is that if an event can be explained, it must be rationally motivated, and that sounds uncomfortably close to endorsing it. To call an action rational, however, is by no means to justify it. Bringing western economies to their knees a few years ago was part of a perfectly rational project on the part of the banks. It sprang from a drive to increase their profits, a motive about which there is nothing in the least insane or impenetrable.

On this logic, the best way not to sound as though you are in favour of murdering soldiers on the streets of London is to see such events as utterly without rhyme or reason, like some baffling Dadaist happening. To concede that they have a motive, however malign, is to invest them with a dignity one feels the need to deny them. British intelligence, one assumes, was well aware some years ago that the IRA had rational grounds for its actions, however reprehensible it may have judged them. They weren’t just killing out of boredom or bloodlust. The popular press, however, preferred to present guerrillas as gorillas – as psychopaths and wild beasts whose actions were simply unintelligible.

There are at least two problems with this strategy. For one thing, if you deny your enemy any shred of rationality, you come perilously close to excusing him. To be bereft of reason, like a baby or a squirrel, is to be morally innocent. This is why barristers do not usually accuse those they are prosecuting of being dangerous lunatics. For another thing, you can kiss goodbye to any hope of victory over your foes. If they do things for no reason at all, it is hard to see how you can defeat them.

After the Boston bombing a few weeks ago, a CNN anchorman asked a so-called expert whether there was anything in the background of the alleged bombers that might help to explain their actions. Unsurprisingly, the expert didn’t reply: “Yes, there is, actually, it’s called western foreign policy.” Instead, he jawed on about the possibility of early childhood trauma. If political motives are inadmissible then psychological ones will have to do instead. Maybe these two young Chechnyans were dropped on their heads as infants, or rudely yanked from the breast.

It is not true, as 19th-century liberals such as George Eliot and John Stuart Mill tended to believe, that to understand all is to forgive all. On the contrary, to place an action in its context may be to deepen the guilt of its perpetrators. Appeals to context are not always ways of letting people off the hook, a fact of which those who ritually protest that their racist or sexist words were taken out of context seem unaware. Invoking the injustice and humiliation inflicted by the west on the Muslim world will not do as grounds for murder. But neither will invoking the necessities of the so-called war on terror do as a justification for massacring the innocent.