What do you call someone who hates mankind?

Someone who hates humanity would be called a misanthrope.

Misanthropy is defined as generalized dislike, distrust, disgust, contempt or hatred of the human species or human nature. A misanthrope is someone who holds such views or feelings.

The previous answer, "Nihilist" argues that life is without objective meaning, purpose, or intrinsic value. Moral nihilists assert that morality does not inherently exist, and that any established moral values are abstractly contrived. Therefore, nihilism is not the appropriate Word.