They gathered in front of politicians their houses in large groups, sometimes even with torches, hoping to scare even the kids inside the house.
They sent myriad of death threats.
Totally not extremist.
(Do not take notice that China vaccinated their own people though, or the cognitive dissonance might be too much).
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1. https://www.rferl.org/a/russia-china-covid-disinformation-ca...
So yes, the alt-right is an anti-democratic extremist group.
As for anti-vaccine groups, their contribution to the death toll of past pandemics is no secret to any moderately informed person.
As for what happened on January 6th, it was in practical terms a concentrated bit of aggressive protest theater, and far from anything seriously resembling an attempted coup. You'd have to be deluded by ideology to call it something so serious. For example, that event was much smaller than the enormous amount of government property damage and calls to topple governments made by a much larger number of people during the earlier Floyd protests across the US and other countries. Would you call those extremist too?
In any case, by naming the most radical actions of a certain subset of a wider belief system as a reason for considering all aspects of that wider belief system as extremist and worthy of banning, you're just another garden variety autocratic monkey at heart, looking for ways justify banning whatever concept doesn't fit your tribal identity.
By your ridiculous logic, any belief system could be justifiably banned because in some ambiguous way, it's "responsible" for the specific activities of certain people who hold to its most extreme version, though the two things (a wider system of beliefs and specific people's active choices) are separate.