I'm about 200 posts late but on South Park: it was huge when I was in school and especially with boys who were into nu-metal/Eminem and Jackass, and I think they all have a nihilistic cynicism at their core and they look like cruelty as entertainment.
I think a lot of this stuff is more sophisticated and nuanced than it seems on the surface. But in high school the joke was not "Cartman's homophobia is abhorrent" the joke was "haha cheesy puffs haha gay"
Twenty years ago a generation of teenagers were given this as entertainment and now they're at an age where they're in jobs and roles of significance. I don't think it's a surprise that the alt-right is growing the way it is in 2020, the roots were planted at the right time.
I'm sure there's a lot in that, and I recognise the stereotypical South Park superfan, but I don't think the causal link between that and the rise of the alt-right is all that straightforward. For one thing, there's certainly a strain of nihilism in various elements of the fringe left, too - tankies, I guess, and the Putin-fetishists and other unpleasant weirdos who exist in the murky red-brown hinterland. For another, pure nihilism is ultimately apolitical, because even the most bloodthirsty ideologies - colonial imperialism, Nazism, Stalinism - ultimately have to stand
for something, as well as
against something else, and nihilism by definition can't actively be in favour of anything, except perhaps immediate personal ego-gratification (which is why I think Trump has become the figurehead for an ideology without actually being ideological himself).
Then there's the extent to which the alt-right isn't so much a new phenomenon (granted that it's taken a form that could only have been enabled by the internet) as a revival of a type of conservatism that the Republican party moved away from during the Reagan/Bush/Bush years, and probably before that to some extent, when it prioritized The Market above all concerns about culture. So now you have all these kids and young adults - and some not-so-young adults - trading in memes all about WWII, European colonialism, the Crusades and even ancient history - "the Chad Roman Empire vs the Virgin Ottoman Empire". Researching their ancestry and talking earnestly about Y-chromosomal haplogroups, genes for lactase persistence and the Bell-Beaker Culture. Luka's mentioned influential right-wing Instagram accounts that are full of photos of Greek sculptures and gothic cathedrals. The Spengler revival.
So if nihilists are people who don't care about anything, then these guys care very, very deeply about things that they think the left hates and that normie, neoliberal conservatives are indifferent to.