The word extremist is most perplexing, specifically in how it has been bandied about in the past to describe groups of quite excusable or innocent viewpoints. For example, the early Christians were considered extremists, their views of equality and unrepentant all-inclusive love being considered counterproductive to firstly the Jewish state and more seriously to the all-powerful Roman empire. This is an example of a peaceful fringe threatening the constructed inner guiding principle, eating away, slowly but assuredly, at the very core ideals of a studiously structured political or religious hierarchy.
The collapse of every colonial system has produced specifically focused extremists, looking to promote positive independence, in their own very individual format quite naturally, once the oppressors have been removed from power, by negotiation, force of arms, or some combination of the two.
Pacifists might easily be considered extremists, especially during periods of strife, the recognition of the conscientious objector is a very modern happenstance, prior to that realization the passive resister suffered punishment as mere common criminals, including imprisonment and flogging, often treated equally harshly by fellow inmates as warders.
Was a time individuals with odd ideas were considered quaint, touched, eccentric, possibly mentally challenged, but still seemed able to function on the edges of society, perhaps suffering scorn, laughter, ridicule, but strangely protected as special, much like the fools of ancient sovereigns.
The questions must be asked, when did acceptance of the norm became overriding human necessity, when did holding an opposing viewpoint, however quirky, became a threat to the common good, whatever that particularly banal phrase night mean. Seems a very modern occurrence, perhaps when the muscle available to show a divergent opinion became quite so perilous to the majority.
I would but remind that most advances in human development, learning, were rooted in profoundly differing perspectives, egregiously persecuted at the time to the point of outright condemnation.