The slide from civilisation to barbarism is shorter than we might like to imagine. Every violent crime taints the ideal of an orderly society, but when that crime is committed against the people who are peacefully selected to write the rules, then the affront is that much more profound.

The killing, by stabbing and repeated shooting in the street, of Jo Cox is, in the first instance, an exceptionally heinous villainy. She was the mother of two very young children, who will now have to grow up without her. It is also, however, in a very real sense, an attack on democracy. Violence against MPs in Britain is mercifully rare. Only three have been killed in recent history: Airey Neave, Tony Berry and Ian Gow, all of them at the hands of the Irish republicans. Two others, Nigel Jones and Stephen Timms, have been grievously wounded, the latter by a woman citing jihadi inspiration and rage about the Iraq war. Whatever the cause, an attack on a parliamentarian is always an attack on parliament as well, which was as clear in Thursday’s case as any before.

Here was the MP whom the citizens of Batley and Spen had entrusted to represent them, fresh from conducting her duty to solve the practical problems of those same citizens in a constituency surgery. To single her out, at this time and in this place, is to turn a gun on every value of which decent Britons are justifiably proud.

Jo Cox, however, was not just any MP doing her duty. She was also an MP who was driven by an ideal. The former charity worker explained what that ideal was as eloquently as anyone could in her maiden speech last year. “Our communities have been deeply enhanced by immigration,” she insisted, “be it of Irish Catholics across the constituency or of Muslims from Gujarat in India or from Pakistan, principally from Kashmir. While we celebrate our diversity, what surprises me time and time again as I travel around the constituency is that we are far more united and have far more in common with each other than things that divide us.”

What nobler vision can there be than that of a society where people can be comfortable in their difference? And what more fundamental tenet of decency is there than to put first and to cherish all that makes us human, as opposed to what divides one group from another? These are ideals that are often maligned when they are described as multiculturalism, but they are precious nonetheless. They are the ideals which led Ms Cox to campaign tirelessly for the brutalised and displaced people of Syria, and – the most painful thought – ideals for which she may now have died.

The police are investigating reports that the assailant yelled “Britain First” during the attack. If those words were used, this would appear to be not merely a chauvinist taunt, but the name of a far-right political party, whose candidate for City Hall turned his back in disgust on Sadiq Khan at the count, in sectarian rage at a great cosmopolitan city’s decision to make a Muslim mayor. The thuggish outfit denounced Ms Cox’s death, as it was bound to do. But their brand of angry blame-mongering could very well serve to convince particular individuals – especially those who are already close to the edge – that some people are less than human, and thus fair game for attack. The rhetoric of western racism and Islamophobia is the mirror of the ideology with which Isis and al-Qaida secure their recruits and that persuades them to strap explosives to themselves, and die in order to kill. It might be especially powerful in Britain, at a time when divisive hate-mongering is seeping into the mainstream.

We are in the midst of what risks becoming a plebiscite on immigration and immigrants. The tone is divisive and nasty. Nigel Farage on Thursday unveiled a poster of unprecedented repugnance. The backdrop was a long and thronging line of displaced people in flight. The message: “The EU has failed us all.” The headline: “Breaking point.” The time for imagining that the Europhobes can be engaged on the basis of facts – such as the reality that a refugee crisis that started in Syria and north Africa can hardly be blamed on the EU, or the inconvenient detail that obligations under the refugee convention do not depend on EU membership – has passed. One might have still hoped, however, that even merchants of post-truth politics might hold back from the sort of entirely post-moral politics that is involved in taking the great humanitarian crisis of our time, and then whipping up hostility to the victims as a means of chivvying voters into turning their backs on the world.

The idealism of Ms Cox was the very antithesis of such brutal cynicism. Honour her memory. Because the values and the commitment that she embodied are all that we have to keep barbarism at bay.