The killers got their bloody hands on the front page first, but they struggled to keep the public's attention. On Friday, the focus moved to Lee Rigby, the man they killed, and the family he left behind. It was his face that stared from page one, the sobbing of his wife heard on the radio news.

Even on the previous day, when the victim was still nameless, the killers were not the stars of the spectacle they had scripted and staged. How galling it would be for them to know that the person attracting the most intense interest was not the men with knives, but Ingrid Loyau-Kennett – who had voluntarily stepped off a bus to insert herself in a lethal situation that she could so easily have avoided, armed only with a Brownie leader's knowledge of first aid. She spoke calmly to the murderers, very possibly preventing further bloodshed – an act of such quiet heroism it astonishes as much as it inspires. It was her, not them, we wanted to know about. If she is not included in the next honours list, then Britain's gongs are more pointless than their most damning critics assume.

Even at the moment of highest drama, as one of the men addressed an amateur camera, his hands drenched red, he did not dominate the scene. Watch it again and your eye goes to two women, unhurriedly walking past him as he speaks of horror and violence, one of them wheeling a shopping trolley. In its own way, it was a peculiarly British moment, surreally recalling the old Morecambe and Wise sketch that had Eric stride across the back of a busy stage, wearing a coat and cloth cap and carrying a shopping bag, as if oblivious of the mayhem around him. That the two women were black, while Loyau-Kennett spoke with a French accent, only completed the tableau of modern, plural London: superficially unrecognisable from the London of 1940, but still a city that knows how to keep calm and carry on.

The behaviour of these women raises a challenging question for the rest of us: when killers strike in this way, should we listen to what they have to say? Or should we walk on, pretending we can't hear?

Judging by our responses to Woolwich and comparable acts of violence, the truth is we don't know. If you were kind, you would say we are confused. Less charitably, you'd say that we are guilty of double standards and hypocrisy. It seems we're ready to listen when we have some sneaking sympathy, not for the act itself, but for the cause it seeks to highlight. But when we find the killer's motive as repugnant as his action, we put our fingers in our ears.

A useful comparison is with the case of Anders Breivik, who in 2011 planted a bomb in Oslo that killed eight people and who went on to murder another 69, mostly teenagers, on the island of Utøya in Norway. He did not spread his message via bystanders' cameraphones, but through an 1,801-page manifesto that denounced what he saw as the evils of mass immigration and multiculturalism.

At the time there was no shortage of voices on the right rushing to denounce what Breivik had done, before suggesting he was voicing a widely felt sentiment, adding that perhaps a frank conversation about the excesses of diversity and the alienating effects of globalisation and migration was overdue. As I wrote at the time: "To listen to it, you'd think Breivik had simply wanted to start a debate, that he'd perhaps written a provocative pamphlet for Demos, rather than committed an act of murderous cruelty."

Some shook their heads ruefully, sadly noting that they had long warned such violence would be the result of the headlong rush to a multicultural, rainbow-hued future.

Liberal and left opinion knew what it thought of such talk. It was wrong to accord Breivik's warped beliefs such a respectful hearing. Airing his ideas this way was to reward his massacre, surely providing an incentive for others to repeat the slaughter. His actions should be treated as murder, plain and simple. To respond by debating his grievances was to cede him, and violence itself, too much power.

Yet when the killer's cause is the matter of western intervention in Muslim countries, it seems some left voices find their previous fastidiousness has deserted them. Cue a BBC interview with Ken Livingstone, who spoke so powerfully after the 7 July bombings in London. Now, he linked Woolwich to Iraq, Afghanistan and the Israeli occupation of the West Bank. Enter the Stop the War coalition, whose statement on Woolwich similarly made the connection with "western foreign policy in the Middle East and south Asia", ending with the declaration that events had proved their position "absolutely right".

Be in no doubt, Livingstone and the anti-war movement would be appalled if their arguments were played back to them in reverse. Imagine what they would say to the claim that Breivik's terror vindicated the old rivers-of-blood warnings, predicting that decades of multiculturalism would end in disaster, and now it was time to change course. Consider their reaction if the right had seized on the bombing of the Admiral Duncan pub in 1999, casting it as the inevitable result of a liberalisation of gay rights that was bound to radicalise a certain young male demographic and that therefore a policy shift was in order.

Of course they'd have rejected such logic utterly. But if it's wrong for the right to seek vindication in acts of brutal violence, then it's surely wrong for the left to do the same. Nor is it any good for the latter to say, "we're not justifying, we're simply explaining": the right said the same about Breivik. Nor can they claim theirs is no more than a cold, analytical judgment, merely forecasting rather than endorsing the logical consequences of a current course of action. Their opponents could and did say the same about multiculturalism after Breivik.

As it happens, I too once made the case that the war in Iraq would only fuel more terror on our own soil. But what happened in Norway has made me hesitant to use that argument any longer. For now we know that there are minds twisted enough to be provoked to kill by any policy they despise. If you believe western foreign policy is wrong, then argue that case. But don't rest your argument on the threat of blowback violence against us. For as we have learned at great cost, in today's world horror can come from any direction.

Twitter: @j_freedland

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