Answer: C) All people have a wicked side.
Step-by-step explanation: In literature, the theme is the underlying message of a story, it is what critical belief about life is the author trying to convey in the writing of a novel, play, short story or poem. Usually this belief, or idea, is universal and transcends cultural barriers. In the given sentence from Edgar Allan Poe's "The Black Cat" we can see that it supports the theme of all people having a wicked side, because the speaker says that he had an "unfathomable longing" to do wrong.