Extremism

Can all people who hold extreme views be fairly described as "extremists"? Extremism is a slippery concept. Its connotations are perjorative but at a glance, it can be difficult to see what differentiates extremism from more acceptable forms of fringe belief such as radicalism. Is it possible to be an extremist in the name of a morally worthy cause? and how should we deal with extremists in our midst?