*extremist:
someone who goes to extremes or holds extreme opinions (it refers to someone or something that isn't moderate or centrist, but it may change overtime);
people or groups that are labeled extremist are considered possibly immoral, potentially or actually dangerous, and certainly wrong.
The label extremist is typically applied by an outsider, but people sometimes use it of themselves to describe their zealous dedication to a cause.
*fanatic:
often has religious associations
fanatical acts or words were thought to result from possession by a demon or deity
fanaticism suggests a crazed compulsion that puts someone beyond the reach of reason
fanatic also can be used in a weakened sense to describe someone whose behavior is zealous but within the normal range: a comic-book fanatic (abbreviated form: fan)
來源：May I Quote You On That
