Confession time: I like Douglas Carswell. Sure, I abhor everything Ukip stand for: they’re a millionaire-funded party advocating privatisation and tax cuts for a thriving wealthy elite, encouraging struggling Britons to direct their fire at immigrants rather than financiers, poverty-paying employers or tax avoiders. But Carswell is a charming and thoughtful bloke more interested in technology-based libertarianism than kneejerk rightwing populism.

The son of a doctor who helped pioneer the treatment of HIV, he was clearly upset at Nigel Farage’s contemptible pre-election attempt to tap into resentment of foreign-born HIV-positive patients. His libertarianism, if implemented, would be nothing short of a social and economic disaster; but his critique of crony capitalism – of the fusion between corporate interests and the state – is one many on the left could easily identify with.

He has written about his unpleasant experience at this week’s anti-austerity protest. So we’re clear, the way he was reportedly treated was out of order. Protest is a basic democratic right, won at great cost by our ancestors. It is often passionate and angry, be it about cuts to public services or opposition to the ban on fox hunting. It includes civil disobedience, employed by protest movements ranging from the suffragettes to tax justice campaigners.

If a disabled sufferer of the bedroom tax angrily accosted Iain Duncan Smith, it would not be the place of privileged me to tut at them. But subjecting Douglas Carswell to what was undoubtedly a frightening and upsetting experience was totally wrong, and it is difficult to see what those involved hoped to achieve.

However, his attempt to use the incident to make political gain – and resort to sweeping generalisations about the left – cannot go without a response. “Austerity, according to the left, is the ultimate evil,” writes Carswell. Well, no, actually. I suspect most on the left, if asked for “ultimate evils”, would opt for, say, genocide, war or murder.

Expecting people to pay for a crisis they had nothing to do with; slashing social security for low-paid workers and disabled people while many of the richest avoid tax on a grand scale; forcing predominantly disabled, impoverished people to pay money for a “spare bedroom”: the “ultimate evil”? No – but profoundly unjust, and certainly worth protesting about.

It is “intolerable” for the left that the Conservatives have won the first general election in nearly a quarter of a century, Carswell suggests. The logic of Carswell – and others – is that if a party wins a general election, then the whole population must submit and accept their legislative programme without so much as a whimper. An elected dictatorship, if you will. Even if that government is pushing one of the most radical rightwing agendas in postwar British history after receiving the support of a quarter of eligible voters – no, we must silently accept everything, from the dramatic curtailing of the trade union movement to the abolition of the Human Rights Act.

“The left,” concludes Carswell, “is no longer driven by either logic or reason.” And there we have it: the actions of a tiny group of individuals is used to smear the millions of people self-identifying as belonging to the left. When George Galloway was violently assaulted – rather than simply yelled at and jostled – I did not make sweeping generalisations about the British right or uncritical supporters of the Israeli government.

When the English Defence League march against Muslims, I may well point to an atmosphere of anti-Muslim prejudice sadly fuelled by people of all political persuasions, but I do not portray them as representatives of the entire British right.

When I receive violent threats calling me a “godless faggot Marxist”, promising to “break every bone” in my body, I do not suggest that the right as a whole is somehow complicit or responsible. This, sadly, is Carswell’s approach.

Carswell blames a “persistent portrayal” of Ukip as “beyond the pale” by “leftwing newspapers and news programmes”.

Perhaps he means, say, calling Farage a “snarling, thin-skinned and aggressive” man surrounded by “wrong 'uns”. Oh, that was Patrick O’Flynn, a Ukip MEP and one of Farage’s closest associates.

Whoops. If Carswell was upset about Farage’s gutter comments about HIV, perhaps he was also uneasy about Farage claiming that people were right to be concerned about having Romanian neighbours.

In any case, what ingratitude on the part of Carswell: presumably by “leftwing news programmes”, he means the BBC, which appears practically to have made Farage a permanent member of staff. Farage has appeared on Question Time more times than anyone else since 2010.

No, Carswell should not have been subjected to his upsetting experience. But if he, or anyone else, believes that people are going to stop protesting about the living wage, the housing crisis, workers’ rights, tax justice, public services, climate change, or whatever, then they will be sorely disappointed.

Yes, the left needs a message of hope, of optimism rallying around a coherent alternative that can inspire. But if the likes of Carswell try to smear the left as a whole – well, then we need to speak up.