There are plenty of left-wing subreddits with equal measures of vitriol. Somehow they avoid earning the same opprobrium that the right-wing ones do. This apparent double standard creates the appearance of bias.
If there are plenty, and you are familiar with them, please name a few. It's not possible for us to "investigate on our own" if we don't know what you are considering "equal measures of vitriol".
/r/anarchism and /r/socialism are two that often advocate violence. The latter is dominated by "tankies", which is a term for communists who love Stalin, gulags, and tanks. They frequently take a "kill 'em all" approach. They are also often mocked by other lefties on Reddit, so please don't assume their opinions are popular.
/r/anarchism is a slightly more complicated case. They do advocate violence in a "bash the fash" way. But their intended victims are often those who advocate for genocide, ethnic cleansing, and other racist or misogynistic methods. Their violent earnestness is also often mocked by the rest of political Reddit.
/r/LeftWithoutEdge was a sub formed for serious discussion for lefties without resorting to violent speech. It doesn't get enough traffic so I urge any leftists / anarchists here to visit it occasionally.
Edit: I should add that one reason /r/anarchism and /r/socialism are still on Reddit is because they took the warnings of the admins seriously. /r/anarchism dialed their rhetoric back after being told they were on the verge of being banned. Most of the banned right wing subs ignored the warnings, and often doubled-down.
I was responding to a comment that conflated the worst excesses of admittedly terrible communities on Reddit with the whole of the 'modern right.' My goal was to explain the thinking of those who are accusing Reddit of systemic bias against the right. Their assertion is that subreddits advocating violence against right-wing activists are tolerated, while right-wing subreddits are condemned for similar infractions. Whether this is objectively true I do not know. In fact, my interest in politics is minimal and I should probably leave these discussions alone in future. I hope Hacker News does not devolve into another online community overrun with political arguments and petty antagonism.
You'll note that I said that there was an 'apparent' double standard. If may not be apparent to you, but it certainly is to some. You seem like you spend more time on the political parts of Reddit than I, so I'm sure you're well equipped to investigate this yourself.