That it took a matter of hours for the terrible events on London Bridge to become part of the election war is as depressing as it is sadly inevitable.

However impassioned and however grim the pleas – including from the victims’ families – for this shocking act of murder not to be politicised, it is impossible for an incident of this magnitude to happen during an election campaign and not to have an impact.

The public, who are also the voters, wish to find someone to blame, and politicians will do all they can to ensure that they are not that person. That’s just how it goes.

But it doesn’t have to be quite as vomit-inducing as it has been. It doesn’t, in short, have to be Boris Johnson’s way.

Politicians will always co-opt tragedy. But only Boris Johnson will do it with a smile and a joke.

It is not especially shocking that a prime minister of a party that has been in government for nine years should seek to blame a terrorist murder on decisions made by the previous administration. Such shamelessness is common in politics.

But only Boris Johnson could appear on television, barely 48 hours after the murder of two young people, and put the blame on a “lefty government”, as though mining for an easy laugh in the Oxford Union.

It is far from the first time this has happened. In 2013, GQ magazine, in its infinite wisdom, made Boris Johnson its “Politician Of the Year” (or, more accurately, its male politician of the year. Women are not eligible).

In front of tables of half-cut journalists and celebrities in black tie at the Royal Opera House, Johnson ambled up to the stage. The event happened in the middle of some rarefied days in Westminster, when Ed Miliband ultimately prevented David Cameron from launching airstrikes in Syria in the wake of shocking pictures of a chemical weapons attack on children.

Johnson played the room for laughs, telling a joke about how Miliband’s vote against military intervention was “a double U-turn”, and so on. It did not go unnoticed by many in the room that this was fundamentally a gag about gassed Syrian children.

It matters, this stuff. Tone matters. The tone set by leaders shapes how a country feels about itself. When Boris Johnson was chief cheerleader for the London Olympics, his influence in making the capital and the wider nation feel so unprecedentedly good about itself should not be underestimated.

But it works both ways. Johnson cannot do serious. He has never been able to. His response to the London riots was an embarrassment.

That he, the prime minister, appears to be incapable of understanding that a murder is not to be played for lowly laughs matters. That some events are not to be deployed for personal advantage in so crass a fashion matters. Tone matters.

Johnson’s behaviour hints at an almost psychopathic selfishness. It is the same selfishness that ends in the absurd situation where the prime minister refuses to answer the simplest question in the world: how many children does he have? We, the public, who are expected to confer our power upon him, are not permitted to know the true depths of the man’s callousness.

We have never been in a situation like this before. No one this profoundly unserious, and profoundly abnormal, has ever been at the reins.

At that awards ceremony, by the way, Russell Brand eventually took Boris Johnson to task. Charles Moore hit back at Russell Brand, Russell Brand was ejected, even Roger Daltrey got involved. Lou Reed was never seen in public again.