Earlier quoted context omitted.
> The problem they describe, violent reaction to opposing views, seem present amongst all sides of the political spectrum and it does not seem limited to extreme opinions. On all sides, sure, but saying that "violent reaction to opposing views" isn't necessarily the result of holding "extreme opinions" seems a bit oxymoronic. Or what exactly do you mean by "violent reaction"?
People often violently overreact to relatively minor social or legal offenses. Like when bad parents who get caught on video get death threats. It's not an extreme opinion that kids should have good parents.
It's not "I believe that fair taxation, transparent government and high standard public services" that is the extreme, abnormal opinion—it's "because you don't, you should be sent to death camp".
In a society where the norm is to value individual security, liberty and bodily inviolability, "violent reaction to opposing views" is necessarily preceded by an extreme opinion. Maybe that opinion only exists in a brief, heated moment, and it's certainly not limited to extremists, but extreme nonetheless.