Disclaimer: I am not a lawyer. Nothing in this post constitutes legal advice. Please consult a lawyer if you have a legal issue.

Accusing someone of being a Natsee or a fascist or a white supremacists is basically protected free speech in the US because it's considered political hyperbole that is typical of online heated political discourse (as long as you're not actively accusing someone of killings Jews or blacks, that is still defamatory). Book author Abigail Shier apparently wants to sue a troon on twitter who accused her of being a pedophile. Sex with children is a crime, so accusing Shier of being a criminal with no evidence is defamatory.

However, such an accusation made against an existing person, even if they are a public figure, is not considered free speech in a lot of European countries that have hate speech laws or Holocaust denial laws. For example, if you accuse someone of being a Natsee in say a country like Germany which has Holocaust denial laws, you are basically accusing them of breaking their local speech laws, and such an accusation could constitute criminal defamation under their laws (which I am not familiar with, this is speculation based on what I've read about speech laws in EU jurisdictions which are more strict than US laws). Accusing someone of being a Natsee in the US might be without consequences, but that's not the case for countries like Germany where such an accusation could be read to mean that you're accusing someone of breaking the law by having committed an illegal speech act. Basically, in Europe you cannot accuse someone of breaking the law without presenting actual evidence that they have actually done so. Hate speech or Holocaust denial is illegal in a lot of European countries, but so is accusing someone a Natsee or a fascist or a white supremacist with no evidence of them actually being one. Bardfinn doesn't realize that his false accusations levelled at his intellectual opponents are not to be as taken as lightly in the EU as they are in the US.

In Europe, accusing someone of being "Nazi" isn't just a random "fighting word" like it is in the US under the First Amendment. America doesn't have a Nazi past the way a lot of European countries do, so in the US you can use "Nazi" casually to refer to someone whose politics you don't like, because it's considered "hyperbolic speech online as part of a heated political debate" and hence protected speech. In Europe however, where many countries do have an actual documented Nazi past, "Nazi" has specific meaning and historical context. When you accuse someone in Europe of being a "Nazi", you're not just expressing disapproval of their politics, rather you are accusing them of historical war crimes, and like any criminal accusation, accusing someone of a crime with no evidence, that is considered criminal defamation in many of the EU jurisdictions Bardfinn has targeted. Again, "unclean hands" comes into play. When you have defamed millions of European Redditors as "Nazis" just because you disapprove of the subreddits they participate in, you have committed criminal defamation against all of them.

"Accusations of fascism and ‘literal violence’ levelled against these women may appear comical, but have real consequences in dehumanising and monstering them, thereby justifying harassment and even violence against them."

https://twitter.com/ProfAliceS/status/1392844410157686784

""Under the European Convention on Human Rights, only the most extreme views akin to totalitarianism or Nazism are excluded from protection. And the judge said that my belief, which is widely shared, and does not seek to destroy the rights of transpeople clearly did not fall into that category.

He said: A person who is free in a democratic society to hold any belief they wish subject only to some modest subjective minimum requirements. Being free to hold a belief means the freedom from being harassed, discriminated against, or having your livelihood taken away from you, if you express that belief. It doesn't mean the freedom to harass others. That was never what my case was about. Gender critical beliefs, and gender identity beliefs, are both protected under the Equality Act, and so too is lack of belief. No one can be forced to profess a belief that they do not hold, like "transwomen are women and transmen are men" and be punished if they refuse. My judgement means that organizations now need to consider whether their policies, encouraged by trans rights organizations, discriminate against people with gender critical views."

Source: https://www.youtube.com/watch?v=jOIKlg71LJc