Unfortunately, I think you are correct about extremism being contagious. As I see it, the basic problem is that quite a few people here (in the U.S.) have it in their head that they can force their views on the majority if they are just violent and persistent enough.
The above obviously relies somewhat on believing myths about voter fraud, etc. I'm sure many here (in the U.S.) believe, at least to some extent) that the election was "stolen." They don't seem to view themselves as having extreme views that are not shared by the majority. People seem to be eager to believe fairy tails if these fairy tails suit their cause. I do not know what the answer is to this wilful self-delusion.
My non-expert impression is that it relates to cognitive dissonance, or more precisely a need to avoid cognitive dissonance. For example, I see people react in a very hostile manner when confronted with historical facts that contradict their beliefs. They end up asserting that anything and everything that contradicts their beliefs is a lie.
en.wikipedia.org