What Extremism Is

 Extreme views are nothing more than those that are not mainstream. For quite a while and even continuing today the green economic movement was regarded as extreme by the fossil fuel and coal mainstream votaries. The result is global warming. Mainstream opinions are driven by social inertia and are not necessarily the best. Instead they simply dominate for sundry reason including the concentration of wealth and power invested in the established order.

The historian Arnold Toynbee pointed out that one of the basic reasons for the failure of a civilization is that once its economy becomes established it is very difficult to get it to change. Not just economics; politics too. He used ancient Greece as an example of a failed civilization with a cause of being unable to upgrade to a Republic like Rome with a unification of all of the city-states into one nation. Without unification they were easy prey for the Roman legions.

Common mainstream views need be upgraded for a society to continue to advance after it has already reached maturity. Usually societies do not. If they are successful the wealthy become globalists so far as possible and neglect upgrades to their own country. I suppose the United States is a contemporary example. Fundamentally anything that would actual change the established social structure is considered extreme. It is the media that presently defines what is or isn’t extreme anyway.
Leading, new ideas are often perceived as extreme in comparison to established mainstream ideas. I believe that some ideas are good and others bad. Mainstream views are safe because they have been around a while. new ideas may be good or bad because they are untested and haven't been implemented. Remember that in the ante bellum south abolitionists were regarded as extremists.