Until mid-2011 I was a member of a small London-based Trotskyist group. Early in that same year, as part of my propaganda efforts on behalf of the group I ended up at a meeting of the Labour Representation Committee, a left-wing faction of the Labour party, where I listened to Jeremy Corbyn deliver a rousing speech on the then raging war in Libya.

From memory, the speech was not so much anti-war, which would have been perfectly reasonable considering talk at the time of Nato intervention, as pro that country's dictator, Colonel Gaddafi. I do not remember the exact contents of the speech – it took place when Corbyn was an obscure backbencher – only that audible groans filled enlightened corners of the hall, including my own, when the left-winger began to reel off what he considered the "achievements" of the Gaddafi regime.

A person cannot conceivably be anti-establishment when they are so willing to line up behind some of the most atrocious \'establishments\' in the world - James Bloodworth

You might call my experience of that day the beginning of my education in the left-wing case against Jeremy Corbyn, who since then has risen from obscure backbencher to likely next leader of the Labour party.

The right-wing case against Corbyn is a straightforward one. Indeed, the conservative press in Britain has constructed an entire vocabulary with which to smear as a lunatic anyone, like Corbyn, who does not accept that the best Britain can hope for is a society where tens of thousands of people a year rely on food banks to survive.

The right's problem with Corbyn is not that he is "unelectable" – in fact the thing conservatives fear most is an electable left-wing politician – rather it is that he talks in tones that make them want to hold their bulging purses a little tighter.

No, Corbyn is striking a chord with Labour activists because in many respects he is correct: a Britain built on finance capitalism and property speculation will never work in the interests of the majority. That isn't Bolshevism; it's the ABC of social democracy. The problem with Labour's so-called modernisers, or Blairites, or whatever you want to call them, is that they appear to have forgotten much of this.

From the television studios at Milbank to the plush conference halls at party conference, at some point over the past 30 years the oppressed began to look a little less oppressed to the policy pedants of the Labour establishment.

The best case against Corbyn is not that he is a wild-eyed socialist, but instead goes back to my initial reminiscence: he is remarkably good at proffering apologetics for dictatorship and tyranny. As well as Gaddafi, Corbyn has in recent years championed/made excuses for Venezuelan autocrat Hugo Chavez, Russian gay-basher Vladimir Putin, the butcher of Bosnian Muslims Slobodan Milosevic and the Cuban dictator Fidel Castro.

He has also worked for Iranian state broadcaster Press TV (home of Holocaust deniers and other cranks) and has referred to fascistic terrorist groups Hamas and Hezbollah as his "friends".

It is this, rather than any desire to make the British economy more like that of Germany – the horror! – which ought to prevent Labour members from voting for Corbyn in the upcoming Labour leadership election. A person cannot conceivably be anti-establishment when they are so willing to line up behind some of the most atrocious "establishments" in the world.

This matters perhaps more today than it did in the past. Large swathes of the world are currently convulsed by war and/or under the boot of dictatorship. The world urgently requires a vocal and internationally minded left – a left which, while recognising imperialist follies such as the war in Iraq, never grovels to religious fascists and whose instinctive reaction to tyranny is one of revulsion rather than reverential talk about the "achievements" of this or that thuggish dictatorship – however "left" the posture of the regime in question

Comrade Corbyn, a nice man who loathes tyranny and anti-Semitism, ends up on platforms lavishing praise on tyrants and anti-Semites - James Bloodworth

Unfortunately, Corbyn's indulgence of tyranny is invariably where politics takes you if you accept the increasingly fashionable view that the US is the world's most malevolent power. In building up the US as public enemy number one, the left must invent disagreements with it – and by extension Britain – to prop up an increasingly tortuous ideological house of cards.

Thus because the US is the beating heart of capitalism, it must always and everywhere be the "root cause" (you will hear that phrase a lot) of the world's problems; and by deduction, any movement that points a gun in its direction must invariably have something going for it.

To agree with David Cameron about, say, the threat from Islamic State (Isis) is to admit there are nastier forces in the world than George Osborne and the Daily Mail. And if this turns out to be true, the main enemy might not be capitalism after all – and thus the illusions begin to melt away.

It may be accurate that, as his supporters like to point out, Corbyn "actually believes in something". And yes, ideology can at times inspire tremendous good. But it can also make a person believe that a goldfish is a racehorse.

This is how Comrade Corbyn, a nice man who loathes tyranny and anti-Semitism, ends up on platforms lavishing praise on tyrants and anti-Semites. And it is how some of the very best now find themselves willing on a man who consistently gives succour to some of the very worst.

The truth is that, however much a Corbyn-led Labour party might claim to be standing up for the most vulnerable, it will always and everywhere be willing to sacrifice the very people it ought to stick up for – the world's democrats, secularists, Jews, gays and women – on the ideological alter of anti-Americanism. This, as I will never tire of pointing out, ought to make Corbyn persona non grata for any principled person of the left.

James Bloodworth is editor of Left Foot Forward. You can follow James @J_Bloodworth and his blog @LeftFootFwd.