The act of labeling a person, group or action as "extremist" is often a technique to further a political goal. There is no sect of Islam that calls itself "Islamic Extremism"; there is no political party that calls itself "Right Wing Extremist" or "Left Wing Extremist". The idea that there is a philosophy of extremism is suspect; that people conclude others are extremists is common.
Rather than labeling themselves "extremist," those labeled such tend to see the need for extreme actions as varying with the situation. Dante said "The hottest places in Hell are reserved for those who, in time of moral crisis, maintain their neutrality" (Inferno, The Divine Comedy). Barry Goldwater said, "Extremism in the defense of liberty is no vice; moderation in the pursuit of justice is no virtue" at the 1964 Republican Convention. "Radical" is a somewhat less negatively-connoted label sometimes used by people or groups to label themselves.
It is a common saying that "One man's 'terrorist' is another man's 'freedom fighter.'" Extremism is perceived through the the mainstream beliefs of the time. History and the mainstream beliefs of a later time may tell a different story.