Are we so different, you and I?” the Emperor wonders of Maximus in Gladiator. “You and I are very much alike,” Belloq tells Indiana Jones in Raiders of the Lost Ark. “I am but a shadowy reflection of you.” “You and me … you and me, we not so different,” a rather poorly drawn Mexican tells Vin Diesel in Fast & Furious 4.

Even when the characters don’t say it, we know it: Pacino and De Niro in the restaurant in Heat. The bit where Keanu Reeves has a clear shot on Patrick Swayze in Point Break and he can’t do it because he’s in too deep and he just has to howl and empty the chamber into the sky (later parodied hilariously in Hot Fuzz). Luke cutting Vader’s hand off in Return of the Jedi.

We share uncomfortable similarities with the very people we prefer to define ourselves against

It’s usually the villain who points it out to the hero, but sometimes things are deliberately muddy. “We’re not so different as you might think,” Truman tells his murderous doppelganger in Capote. “We are not so different, you and I,” George Smiley says ruefully to Karla in Tinker, Tailor, Soldier, Spy.

You and I … we are not so very different. There’s a reason a version of this line has appeared in so many movies down the decades. It tells a psychological truth about our uncomfortable similarities with the very people we prefer to define ourselves against.

Having meandered through that, I must now ask you as a matter of urgency to imagine Jeremy Corbyn in the grubby vest of Vincent Diesel, staring down the barrel of a gun at Mr Tony Blair, who is taunting him: “You and me … we not so deefferent.” Or, if you are the other way inclined, do imagine Blair in the heroic vest, and Corbs as the villainous Mexican.

As defining characteristics of politicians go, an unshakeable belief in one’s own moral purity is surprisingly rare in politics. For whatever reason, it is not believed to occur naturally in the Conservative party. Our dear leaders have many and varied strains of self-belief, of course, which are frequently quite virulent. But that specific and rather terrifying conviction that something MUST be morally good because it is they who are doing it – or that something they have done CANNOT be morally bad because it was done by them – is really a class of two for me, in my lifetime.

The first is Jeremy Corbyn, whose fundamental belief was on show this week that he couldn’t possibly have encouraged antisemitism because of the very person he is. And the second – can you guess what it is yet? – was a chap named Tony Blair.

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You don’t hear much about that guy any more, so I am pleased to offer a refresher course for anyone who became obsessively interested in politics in the past few years and may care to buff up on their heritage. Blair was a guy who believed he was a pretty straight sort of guy. Indeed, those were the very words he used when responding to the media (he detested what we now call the mainstream media) about a bad situation he brought on himself shortly before he took office.

He flatly refused to accept that the thing looking awful might actually be awful. He was actually “hurt and upset” by what the media had written about him. A lot of his supporters thought it was disgusting, what they were doing to Tony – total agenda. Some of this may sound vaguely familiar.

For those of us who already thought Blair was suspect at this early stage, this was just a worrying sign of stuff to come. The faintly messianic act, the belief that media interrogation of what he was doing was evidence of an “agenda”, the bullying dismissals by his henchmen and outriders of legitimate inquiries – these things, those of us who were not Blairistas believed, were things that could end up really mattering.

Not that many Labour supporters wanted to hear it at the time. Indeed, some may now be asking: OK, so what? The guy just thought he was morally right about everything – what’s the worst that could have happened? What’s so dangerous about that?

Well … I don’t want to spoil it for those catching up with the box set. But mistakes were made.

Indeed, this particular form of self-belief became the sine qua non of Blair for me, the thing from which all the mistakes flowed. They were expressions of the trait. It feels vanishingly unlikely that Corbyn would make the same mistakes because of his similar conviction of purity. But he would make other mistakes, potentially grave in different ways. People who believe they are morally infallible always do. To govern is to have your personality relentlessly and unsparingly exposed by events.

That sort of exposure has befallen Corbyn over the past week, while he is still in opposition. Some of his defenders are even the same people who shored up Blair’s worst bits. Chris Mullin, once one of Blair’s best little helpers as a foreign office minister between 2003 and 2005, was this week tweeting: “Sorry to see Jewish leaders ganging up on Corbyn.”

Oh dear, Chris (again). I guess enablers gonna enable. Corbyn still cannot accept the reality that antisemitism has flourished under his leadership, often encouraged by his actions, and certainly by his inactions. He can’t accept it because of the things he thinks he is. This is a failure of the imagination, just as Blair’s inability to countenance faults was a failure of his.

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As the much-praised Avenue Q song goes: everyone’s a little bit racist. It doesn’t go “everyone’s a little bit racist except Jeremy Corbyn”. All of us – to varying degrees obviously – make judgments based on inherent prejudices, frequently without realising. Realising that it’s possible we don’t realise – that’s important, especially in someone regarded as a moral leader, never mind someone who styles himself as one.

None of us should kid ourselves that we’re immune from making mistakes in this department, perhaps terrible ones. Given the history of the 20th century alone, none of us should imagine that we couldn’t fall prey to dark currents, dog whistles, lazy thinking that over time calcifies into dangerous thinking. In Primo Levi’s arrestingly simple words: “It happened, therefore it can happen again.”

As far as the fatal flaw Blair and Corbyn share goes, it’s hard to imagine things not ending badly for anyone in its grip. We know neither can stand the other. But him and him, they are not so very different. In films, the moment that line is said is ideally the moment the hero pulls back from the edge, and starts to redeem themselves. With Blair still unable to accept his mistakes, and Corbyn still convinced of his moral infallibility, escaping the cycle feels difficult. It would certainly be nice. But maybe it’s the sort of thing that only happens in the movies.

• Marina Hyde is a Guardian columnist