Sentencing a criminal to death is considered "extreme," however, many view it as a necessary measure. The American colonists declaring civil war against the British Empire was also "extreme." America's independence day is one of the largest holidays in the US.

Liberal democracy is considered centrist, despite it being a radically difference form of "civilization" than anything pre-Enlightenment. Under liberal democracy, mass-suffrage is a centrist position and the idea of the state as being what defines a people is centrist. Completely contrary to most of human history -- even democracy in Ancient Greece never went so far.

In a sense I'd argue that extremism is nothing more than going against the fray of the prevailing winds of modern society, which doesn't seem to offer all that much to people except the mercantile class.