What did Algis say, 'bout DNA; anyone?
Extremism varies depending upon the source;
Most causes having extreme elements are emotional in nature (isn't everything!).
Emotional reaction to an idea, invariably is more volatile than a considered intellectual reaction, but then, who wants to rally behind some well thought out reasonable compromise giving metered equality to those deserving it over an agreed upon period of time? Not I, ses I!
So, as it is with politicians, the old "any publicity is good publicity (just make sure they spell my name right)" leads the way, and clever leaders of many "causes" use the media to manipulate the public into paying attention to their needs, even if somewhat disturbed by their methods; sober consideration of the facts will always lead to ignoring the extremes on both sides.
Does the "public soberly consider anything? - different ballgame!
The basic difference is with worthwhile causes, there is a core of leadership who are intelligent beings, and who, when faced with endangering inocent bystanders or equivalent, will shift gears from emotional abandon, to intelectual reality, and reel in the more boisterous of their supporters before irreversible damage has been done, allowing the "cause" to bask in the harsh glare of publicity, and lobbying diligently behind the scenes.
In other cases, where real harm is considered appropriate, the emotional fringe has taken over, and the mob is no better than terrorists, who's twisted logic clamours for murder to protect the rights of their group to live in peace; some logic!