"Extremist" is a politically convenient term currently being hijacked by the media and politicians for anything politically or physically violent (traditionally the "terrorists") or simply not following the politics of the western governments. In the 70's/80's it was widely used in the UK as a term for anyone simply not following the mid to right wing political dogma - in particular socialist groups who were branded 'left wing extremists'. Violence not orchestrated by the state was generally attributed to 'terrorists'.
An "extremist" is someone outside the bounds of your personal bubble ...