Yesterday, May 14th, a white supremacist committed a mass shooting in Buffalo NY, killing at least 10. In a long manifesto, he mentioned using 4chan for news since May 2020, and Reddit before that. Here's a screenshot of the relevant page. https://archive.ph/fMNgJ Using the website unddit, we can see all of the comments he ever left on what is universally accepted as his Reddit account. https://archive.ph/L0B1w

One of these comments is on r/ politicalcompassmemes. It seems innocent enough, consisting of one word: "based." But if you go to the comment, you see the whole thread. https://archive.ph/L3p8b Someone explaining what the point of the stimulus checks were, followed by someone saying "As with everything, I blame the big nose." Referencing the antisemitic caricatures of Jews having big noses. This is the person the shooter replied "based" to.

PCM users have said that one comment doesn't mean anything. But there are no comments on any news subreddits, either, in his three years of being a Redditor (as we see from his oldest comment, dated 2019), even though he used Reddit for news until May 2020. He was clearly a lurker. This means that it is completely reasonable for him to have been subscribed to PCM for any amount of time while only leaving the one comment.

So, we know he was in PCM. But did it contribute to his radicalization? Absolutely. In the previously mentioned page from his manifesto, he mentioned being convinced through memes, shitposts, and infographics that the white race was dying out, an intentional effort by the Jews and elites. PCM has also been posting "memes and shitposts" which say this, sometimes explicitly, as early as June 2021. Many of them are comments without many upvotes, but they are clearly being easily found by those who know where to look. The ones in bold are posts that were uploaded after his "based" comment (January 4th 2022).

• “Erasing white people is among the Democrats’ top concerns.” Comment, Double digits (10), 22/6/2021 https://archive.ph/p8Fdp
• “Anti white hate crimes and white genocide are real issues” (The second archive is a screenshot of the post when it was still up). Post, Triple digits (200), 2/12/2021 1: https://archive.ph/dPSbr 2: https://archive.ph/810wR
• “The declining white population in the United States is intentional and planned.” Comment, Double digits (10), 12/8/2021 https://archive.ph/0oBAb
• “LibLeft is going to start killing white people soon.” Post, Triple digits (350), 13/6/2021 https://archive.ph/k8zcy
• “The trucker convoy organizer believes in white genocide? Damn he’s based as fuck.” Comment, Double digits (40), 1/2/2022 https://archive.ph/74cgN
• “Replacement isn’t a theory so much as an inevitably.” Comment, Double digits (20), 17/03/2022 https://archive.ph/y4c7W
• “The green party is trying to end our (white people’s) existence.” Post, Triple digits (760), 23/03/2022 https://archive.ph/xWg3V
• “LibLeft wants to kill white people.” Post, Quadruple digits (3900), 23/03/2022 https://archive.ph/Z4mTy
• “Why is it that these people who are barely one percent of our population have the power to bend our politicians, media, and corporations to their will? …I’m talking about those orange LibLeft Emilies, of course. Who else?” The second highest comment is deleted and has a reply with 90 upvotes that says “I always knew Emily looked Jewish” in case you have any doubts about the rest of the sub’s interpretation. Post, Triple digits (930), 06/01/2022 https://archive.ph/UXTZl
• “Chinese agitators are making Western society more accepting of immigrants.” Post, Triple digits (430), 3/3/2022 https://archive.ph/g1BKg
• “There’s a set of beliefs that gives its believers enormous societal power over others, like there’s a secret global dictatorship they run. …Progressivism of course, what else could I be talking about?” Comment, Double digits (20) (the archive took a few upvotes off), 03/03/2022 https://archive.ph/9Clfg
• “The woke’s sole duty is to destroy Western society and culture.” Comment, Double digits (40), 19/3/2022 https://archive.ph/JgDeK
• Someone made a post about how only whites are only 49.9% of the people under 15 in the US, comments are full of people saying the Great Replacement is real, one’s even saying saying minorities are going to do a literal actual genocide on white people, motivated by Critical Race Theory and the “white oppressor narrative”. Quadruple digits (3350) on the post, triple digits (160) on the highest comment. 14/05/2022 https://archive.ph/6dygc
• “Critical Race Theory wants literal white genocide.” Post, quadruple digits (5960), 11/5/2022 https://archive.ph/AfjBe

This is the exact same stuff white supremacists on 4chan say. And this is only the stuff where people are explicitly endorsing the Great Replacement. The pinned post on r/ EnoughPCMSpam https://archive.ph/8iXu3 has the full PCM Alt-right Archive, it's very long and has much more than just Great Replacement, any and all of which could easily radicalize someone to the far right, especially if they were also hearing the same things elsewhere.

I think it's likely that after seeing a few PCM memes reposted to 4chan (PCM has established itself as part of the online fashosphere, a handful of places where far-right memes make the rounds https://archive.ph/53ocW), he asked about the colours over the wojaks, and was directed to PCM. He obviously was in PCM for a while, but I think this is the most likely way he found it. If this is the case, I'd bet it happened in June 2021 at the latest, because 28 posts and comments were added to the Archive during that month, which is 30 days long. PCM was in a particularly extreme mood that month.

tl;dr PCM helped radicalize a mass shooter, time for Reddit to ban it.

EDIT: added an archive I missed (the last one) and highlighted the posts we know with 100% certainty were posted while he was in PCM. Here's a pre-edit archive of this post in case you don't trust me: https://archive.ph/MNNTG