The nightmare has happened. The worst of men is elected prime minister. The hardest of times lie ahead. Unfit in every way for any kind of office, Boris Johnson takes up the reins of absolute power bestowed on any leader with such a majority.

This blow has fallen on a country ravaged by a decade of decay in the public realm and a stagnant economy. We have become an embarrassment abroad. Brexit was the wicked weapon Johnson used to dethrone his last two leaders in order to lever himself into their place. Reckless in everything but personal ambition, he has trapped us into the worst Brexit imaginable, withdrawn from the EU next month and out with a disaster of a “deal” next year.

Five crucial years will be lost in the fight against the climate crisis. In search of deals, he will bend to every interest, every lobby, every fossil fuel and fracking pusher, hiding behind his empty 2050 zero emissions pledge. The shriveling of every service is cemented into his budget plans: enough to stop outright NHS collapse, not enough to get schools or police back to 2010 levels, and everything else destined for never-ending decline. Expect no sudden change of heartlessness. We Cassandras have wrung our hands and howled out loud warning of rising poverty, homelessness, collapsing legal and social care systems, living standards in reverse. Yet people voted for all this woe.

Who is to blame? There are the lies of the extreme Tory press, echoing around all media – but Labour always faces that injustice. It is the rough sea that any leader must try to navigate. Unabashed by the valiant Full Fact and other fact-checking organisations, Johnson found he can repeat a lie a thousand times with utter impunity, no one to stop him except the people – and they have preferred the lie.

They are not deceived: they call him untrustworthy. Anyone listening hears his plans for revenge on all who thwarted him: he will dilute the powers of the supreme court for defying him. He threatens Channel 4 and the BBC with malevolent “reviews”. Beware any civil servant or regulator who gets in his way, as he curtails the right to judicial review and threatens to “update” the Human Rights Act. The pound surges as City folk fear paying higher tax more than they fear a bad Brexit crippling the entire economy.

Given the worst choice in history, the public preferred him to his opponent. How bad did Labour have to be to let this sociopathic, narcissistic, glutton for power beat them? That’s the soul-searching question every Labour member, office-holder and MP has to ask.

‘How bad did Labour have to be to let this sociopathic, narcissistic, glutton for power beat them?’ Boris Johnson delivers a victory speech to Tory party members. Photograph: James Veysey/REX/Shutterstock

Labour was disastrously, catastrophically bad, an agony to behold. A coterie of Corbynites cared more about gripping power within the party than saving the country by winning the election. The national executive committee, a slate of nodding Corbynite place-persons, disgraced the party with its sectarian decisions. Once it was plain in every poll and focus group that Corbynism was electoral arsenic, they should have propelled him out, but electoral victory was secondary.

Should we laugh or cry at Corbyn’s announcement that he wouldn’t stand for another election? He should have gone before dawn. Any possible or impossible successor will clear out that Len McCluskey clique – Karie Murphy, Seumas Milne, Andrew Murray and others who propped up the old fellow to secure their own power base – with results worse than Michael Foot. Watch them try to divert blame onto “Corbyn-disloyalists”, remainers and ”Blairites”.

Corbyn is not an amoral man. He can never tell a lie: pretending to watch the Queen’s Christmas message in the morning showed he’s not used to fibbing. He is a man without any qualities required of a leader, mental agility, articulacy, strategy, good humour or charisma.

Yet his legacy is of historic importance: he did this country profound, nation-splitting, irreparable harm. Had he led his party and the unions full tilt against Brexit, the narrowly lost referendum could have been won. But he and his cabal refused outright: when beseeched, they said they were too busy with May’s local elections. He wouldn’t share any remain platform. Festering Bennite 1970s ideologies blinded his sect from seeing Brexit was the far right’s weapon of buccaneering destruction. He could have saved us – but he obfuscated.

Corbyn came weighted with baggage too heavy for a Hercules to shift: the IRA, the Hamas friends, Venezuela. But antisemitism was accusation he could not shift. I am certain he sees no stain of it in himself, refusing to comprehend it, and so could not apologise. Failure to purge every case left candidates on the doorstep dumbstruck when anyone said “I can’t vote for an antisemite”. And remember that early refusal to sing the national anthem? Voters’ first impression was his deep-seated aversion to expressing patriotism.

The campaign was chaotic, all front-bench talent banished for fear of outshining the leader. Toe-curlingly bad performers and insignificants were punted up as loyalists, while serious heavyweights Keir Starmer and Emily Thornberry might as well have been shut in Johnson’s freezer. Even John McDonnell, better by far than Corbyn, was largely kept from the cameras. Corbyn’s sectarian grudges prevented any effort to heal the party’s rift, leaving immense talent wasted on the back benches.

Here’s the real tragedy. The manifesto was essentially magnificent. The vision was of a country freed from years of darkness with green investment, growth in places that most need it, salving the many wounds of marrow-deep cuts, restoring pride in the public sphere and hope in a future that was absolutely affordable. Why should we not tax and spend the same as similar north European countries? But if socialism is the language of priorities, these were lost in a profusion of never-ending promises too easily mocked. The political landscape was never prepared, soil untilled, last-minute policies falling on stony ground. Where was the simple five-point pledge card?

Credibility is everything and Corbyn lacked it like no other. Without credibility all was lost. Think on it, every Labour member. It will be a long, long road up from such a fall. There will be days to consider hope: today is for confronting reality.

• Polly Toynbee is a Guardian columnist