One who uses fear as a weapon of power

One word substitute is Terrorist.

Terrorist: a person who uses unlawful violence and intimidation, especially against civilians, in the pursuit of political aims.
Militant: favouring confrontational or violent methods in support of a political or social cause.
Extremist: favouring confrontational or violent methods in support of a political or social cause.
Anarchist: a person who believes in or tries to bring about anarchy.