Not sure??? In what way at all is that left-wing?
It casts the police as the villains, and a black man as the victim. That meme is more popular on the left than on the right.
There was a theory that AIDS was created in a government lab, to exterminate blacks and gays.
There is a theory that Big Pharma has a fast and cheap cure for [whatever disease is being discussed at the moment], but They are suppressing it, because perpetually treating a disease is more profitable than permanently curing it. This is not exclusively left-wing, but the anti-capitalist element makes it more popular on the left than on the right.
A lot of well educated liberals are anti vaxers along with conservatives.
Very unfortunately I know/knew a few very well-educated, very left people who are/were 9/11 Truthers of the “Wag the Dog” variety. The oil argument for the invasion of Afghanistan as mentioned above tied into it.
I didn’t realize it was a competition. And if Q-Anon is the benchmark then we might as well ask the mods to close the thread because nothing is as weird as that.
Every time I tried to think of an “exceptionally weird” left wing conspiracy theory, all the ones I could remember have now been adopted by at least the leftmost half of the left wing. Essentially everything bad or undesirable is deliberate: X is genocide, X is cultural appropriation, X is a dog whistle, X is White Privilege, etc., etc.
The gulf of Tonkin incident was fabricated (not that weird I guess but it was considered a bit loony to believe it)
The CIA experimented with mind control.
The CIA funded the Dalai Lama
The FBI tried to kill MLK
NATO and the CIA funded freelance right wing terror groups all over Europe to attack left wing governments.
And all turned out to be true.
Gulf of Tonkin happened on August 2nd but not August 4th.
America strongly assisted Britain in their war with Argentina over the Falklands because Reagan and Thatcher were having an affair.
Gary Hart was set up. (Oh, wait. That one turned out to be true.)
Burning “fossil fuels” like coal and gas will lead to “global warming”. 
Back in the 70’s I knew some people who were convinced that Bob Hope was funding the War in Vietnam, just so he could have an audience.
From the OP:
I assume it’s supposed to be “just as bonkers.” And a theory about Monsanto concealing scientific evidence about their product is nowhere in the same league as bonkers as right-wing conspiracy theories that have become nearly mainstream in Republican circles.
That said, the idea that car companies have engines that get 300 mpg, or run on water, and are concealing these engines to help oil companies? I heard that one in college. It was pretty silly. Still not “lizard-people pedophile blood-drinker” silly, but pretty silly.
A large segment of my extended family didn’t bother to vote in 1968, because RFK’s assassination proved (to them) that the big corporations would kill anyone who tried to go against the military-industrial complex. Reform (they said) was impossible, and democracy was a failure.
If it were just that Monsanto is concealing how dangerous its products are, that wouldn’t be particularly bonkers. But anti-GMO conspiracy theories go well beyond that. They posit non-sensical risks, like “transhumanism” (somehow the altered genes in the GMOs will alter your DNA when you eat them). Many of the accusations of “dangers” from GMOs aren’t even wrong. They’re just bonkers. Since anyone with a passing understanding of genetics and biology will dismiss the more ridiculous concerns as the nonsense they are, these theories require not just Monsanto but basically every geneticist and biologist on the planet to be in on it. In that sense, they’re more ridiculous than a lot the QAnon nonsense, which at least doesn’t generally involve violations of well-understood science.
On the other hand, anti-GMO conspiracy theorizing (like anti-vaccine CTs) has spread to the far right, where it usually involves “health freedom” and concerns over bodily purity more than the left wing concerns over environmental damage and anti-coporatism, so it’s no longer really a left wing conspiracy theory, per se.
Remember Jeff Gannon, the “reporter” who was planted in the White House press pool to toss softball questions to Dubya around 2005?
I knew several left-leaning people at the time who insisted that, because there were days when he forgot to sign out in the White House guestroom, he must have been Dubya’s secret gay lover who was spending the night with him in the Lincoln Bedroom.
I knew another far-left person around this time who believed the 2004 flu vaccine shortage was a plot to kill off the elderly, and even claimed that Donald Rumsfeld had given a TV interview where he told seniors it was the most patriotic thing" they could do to die quickly.
Those are the craziest that come to mind.
Ah–I’ve not encountered those. Mostly I’ve encountered things about how genes could spread to wild species, or how certain GMOs could render certain organic practices ineffective, or how certain GMOs could increase use of pesticides, or how certain GMOs could concentrate arable lands in the hands of fewer farmers with readier access to capital, or how certain GMOs could present cancer risks. Maybe I’ve just missed the really wacky ones.
I would if it weren’t behind a paywall. Maybe you can summarize it more.
I think the "CIA (or FBI) killed Kennedy has to be counted as left wing.
'78, but who’s counting?
He didn’t marry Nicole until '85, so they certainly couldn’t frame him for mudering her before they were married, now could they?
And besides, Peter Hyams, Jame Brolin, Elliot Gould and Sam Waterston are still alive and on the streets. Those people on the moon landing cabal better get cracking, or these guys are going to die of old age before they get conspiracied.
What more needs to be added to what I already quoted? Summary: Dangerous viruses are an important part of biome diversity, trying to keep them from killing people is a part of the plutocratic plot to suppress nature, and Bill Gates is a fount of enormous Evil, as symbolized by his conspiring to amass ever-increasing amounts of money by giving it away.
If the Organic Consumers Association website is “behind a paywall”, then they must think I’m a member, or don’t want links to their silliness posted elsewhere. You can easily find their diatribe via a Google search.
Similar bizarre glop has been posted on their Facebook page.