One who takes delight in excessive cruelty

One word substitute is Sadist.

Anarchist: a person who believes in or tries to bring about anarchy.
Sophist: a paid teacher of philosophy and rhetoric in Greece in the Classical and Hellenistic periods, associated in popular thought with moral scepticism and specious reasoning.
Sadist: a person who derives pleasure, especially sexual gratification, from inflicting pain or humiliation on others.
Nihilist: a person who believes that life is meaningless and rejects all religious and moral principles.