a person who advocates or believes in anarchy or anarchism.

a person who seeks to overturn by violence all constituted forms and institutions of society and government, with no purpose of establishing any other system of order in the place of that destroyed.

a person who promotes disorder or excites revolt against any established rule, law, or custom.

noun

agitator , insurgent , insurrectionist , malcontent , mutineer , nihilist , rebel , revolter , revolutionary , terrorist , anti , radical