We should establish some ground rules first. As I write, we do not know for certain what led a stranger to kill Jo Cox. Such barbarity is almost impossible to fathom.

But that does not mean we should ignore the context within which a young MP was stabbed and shot to death in broad daylight. Quite the opposite.

Political violence – and that is what this was, make no mistake – does not happen in a vacuum. That’s not political point scoring, that is reality. To deny it – as some have done over the last 24 hours – is at best naïve, at worst lethal.

So what do we know? We know that Cox’s assailant shouted phrases that included the words “Britain First”. We also know that he had previous online sympathies for other extreme far-right movements.

But more importantly, and what we know for certain, is that Jo Cox was a passionate anti-racist whose life was taken after the most poisonous, hate-filled month in UK politics in living memory.

A few hours before Cox’s brutal death, Nigel Farage unveiled Ukip’s latest election poster. Entitled ‘breaking point’ it features a seemingly endless queue of brown and black males (they are all men as far as I can tell). The message isn’t a subtle one: leave the European Union, or the hordes of foreigners are coming.

Even by Farage’s standards it’s a disgustingly racist piece of work.

The Guardian’s Jonathan Jones compared it to Enoch Powell’s infamous ‘rivers of blood’ speech. But there’s a crucial difference – in 1968, Powell was already on the fringes of the Tory party. He would later decamp to an Ulster Unionist seat in South Down, practically the only place that would have him.

Farage is a national figure in a campaign that could very well win next Thursday. He is not a marginal voice railing against the dying of the colonial light, but rather one of the UK’s popular and recognizable politicians.

Farage can set the political agenda in ways that Powell could only dream of.

The Ukip poster did not directly cause an unhinged individual to take up a gun, but as Alex Massie rightly noted in his powerful response to Thursday’s events,  “When you shout BREAKING POINT over and over again, you don’t get to be surprised when someone breaks.”

The entire Brexit campaign has been one long, shrill ‘breaking point’ dog-whistle gleefully tooted by our right-wing press and politicians. The tragedy is that it has taken the cold-blooded killing of a compassionate MP for most of us to see how toxic our politics has become.

While Cameron, Osborne, et al have stuck rigidly to the economic script, the leave side (with some honorable exceptions) has played what can only be described as the ‘colonial card’.

“Let’s make Britain great again” declares Boris Johnson. And when was Britain greatest? In the good old days of the empire, of course.

Vote leave to “take your country back”. From who? The non-whites, of course.

Calls to reclaim the UK’s prestige have traction in a state where so many have yet to come to terms with the end of colonialism.

Unlike fellow EU nations such as Germany, Spain or Portugal, a sepia-tinged vision of the colonial past is quite mainstream, particularly in England, uniting both elite politicians and an increasingly disenfranchised white working class. The vicious reality of the UK’s colonial expeditions is still a marginal narrative, if it is aired at all.

Indeed, the imperial fantasy of the White Dominions – those British territories with significant settler populations – has been implicitly revived by some leave campaigners who have called for an end to EU migration in favour of attracting new comers from Australia.

Britain reluctantly joined the then European community in the early 1970s. In the immediate postwar years Westminster still believed it could be a singular global player. Anti-colonial movements in Kenya, Indonesia and elsewhere were viciously suppressed. Only the disastrous Suez Crisis publicly exposed the limits of British power in the nuclear age.

Now Farage, Boris, et al are merrily evoking British – and English – exceptionalism once more. It was not surprising that as well as singing about the IRA and German bombers, rioting England football fans in Marseille last week could be heard chanting “F*** off Europe, we’re all voting out.”

‘Great Britain has lost an empire and not yet found a role’, Dean Acheson famously remarked in a 1962 West Point speech. More than 50 years later, the American diplomat’s words have never been so true.

But rather than finding a new role in the modern world, British politics is retreating back into right-wing, imperial fantasies.

This noxious referendum did not murder Jo Cox, but it has done much to fuel the hate that killed her.