The word traitor is thrown about so carelessly in these fraught times that it has been rendered all but meaningless.

In the churning sewer of social media, it is perfectly common to see political disagreement treated as an act of treason. Who can forget the joyous deployment of the slur “quisling” against opponents of Scottish independence during the 2014 referendum?

And if that accusation seemed to stretch hyperbole to breaking point, the 2016 European Union referendum had news for us. To hardcore Brexiteers, a vote to remain in the EU was a vote against the UK’s interests. It was a shameful act of capitulation to unaccountable foreign powers, the dereliction of sovereignty. To vote Remain was to be a traitor.

Kim Darroch. (Photo by Alex Wong/Getty Images)

This application of such a serious charge against people doing nothing more than exercising their democratic rights was, at once, both ludicrous and sinister. Ludicrous because honest political disagreement is not treason; sinister because some, whipped-up by cynical, dog-whistling political charlatans, clearly believe it is.

So, if we could all be a little more careful about the use of the word traitor, that would be a very good thing, indeed.

Which brings me to the man who looks certain to become our next prime minister. I’m struggling, these days, to think of a more appropriate word than traitor to describe Boris Johnson.

Last weekend, leaked documents revealed that the United Kingdom’s ambassador to the US, Sir Kim Darroch, held some entirely unsurprising views about President Donald Trump. In memos sent from Washington to London, Sir Kim described the Trump administration as “dysfunctional”, “diplomatically clumsy”, and “inept”; the president, added Sir Kim, “radiates insecurity”.

There was nothing in the memos published last weekend that should have surprised any of us. We have all seen the behaviour Sir Kim described.

Nonetheless, publication of his remarks was a catastrophe for the envoy, who was – predictably – soon in the president’s sights.

The ambassador was, according to Trump, “wacky” and “very stupid”. Furthermore, the US would no longer deal with him.

Inevitably, Sir Kim’s woes took up a significant chunk of the debate between Boris Johnson and his rival in the Tory leadership contest, Jeremy Hunt, that was hosted by Julie Etchingham on ITV on Tuesday evening.

Asked whether the ambassador had their support, the men gave starkly different answers.

Hunt was unequivocal. Were he to win the leadership contest and become the next Prime Minister of the UK, he would gladly keep Sir Kim in post until his pinned retirement at the end of this year. Johnson refused to offer that support.

It was a grim moment. The man who styles himself a statesmen, as a Churchillian figure, cut the ambassador adrift. The following day – citing to friends Johnson’s refusal to back him – Sir Kim resigned his position.

Utterly blameless in all of this – after all, a key part of his role was to offer clear and unvarnished analysis to his political masters – Sir Kim was thrown under a bus by Johnson. Some statesman.

A chill will have swept across the diplomatic corps. If the man who looks certain to be the next prime minister will not stand by the UK’s people, what point is there in any of them risking candour? Who would offer up any honestly held opinion if doing so might end their careers?

In the aftermath of Sir Kim’s resignation, Scottish Conservative MP Ross Thomson, a man without honour, merrily debased himself on behalf of Johnson. Interviewed by the BBC’s Glenn Campbell, Thomson said “I don’t think you defend diplomats when it’s against the national interest”.

But surely it is always in the national interest to protect those who serve with honour, as Sir Kim undoubtedly has? Or are we really to believe that craven capitulation to Trump should be regarded as evidence of greatness?

Thomson – Johnson’s campaign manager in Scotland – is a hard Brexit crank, of course. He’s part of that band of Tory MPs whose furious insistence that Brexit will be a roaring success despite dire warnings to the contrary marks them out as unthinking ideologues. Poodle Thomson, like his owner Johnson, is of the “believe more” school of politics which is currently hugely fashionable among MPs whose claims run counter to reality.

Could Sir Kim have carried on as ambassador after Trump’s remarks about him?

Even if he’d “believed more”? No, probably not, but there is a principle here and it is not an unimportant one.

Sir Kim was entitled to the public support of the man who wants to lead the UK. He was entitled to hear Boris Johnson affirm his confidence in him. He was entitled to expect the PM-in-waiting to say that it was not for foreign leaders to decide who could and could not serve the Government of the United Kingdom.

Instead, Johnson showed precisely the moral weakness that makes him unfit for the great office he is soon to hold. He showed the world that, when things get tricky, he can be depended upon to be undependable.

Is a leader who will happily throw a senior diplomat to the wolves a traitor? If we’re still at all concerned with unfashionable concepts such as decency and integrity then, yes, he is.

Lickspittles like Ross Thomson may be able to live with the troubling idea that it was not in the national interest to defend Sir Kim Darroch. I’d argue that it was not in the national interest to do anything other.

Johnson is odds on to defeat Jeremy Hunt in the Tory leadership contest. We will learn if he is to become our next prime minister in nine days time.

If he does win – and, of course, he will win – then we will be living under the government of a man who cares about nothing other than the satisfaction of personal ambition. Boris Johnson’s loyalty is to Boris Johnson, not to the nation he claims he wishes to lead.

Usefully, there is a word to describe perfectly a prime minister who puts himself before his country and those who serve it.

Euan McColm: Ambassador episode shows shameful Boris is not fit to lead