One who thinks that human nature is essentially evil

One word substitute is Cynic.

Agnostic: a person who believes that nothing is known or can be known of the existence or nature of God.
Cynic: a person who believes that people are motivated purely by self-interest rather than acting for honourable or unselfish reasons.
Sceptic: a person inclined to question or doubt accepted opinions.
Misogynist: a person who dislikes, despises, or is strongly prejudiced against women.