The Black Worm

Buddy still did not know for sure if the black worm really existed in the form that he imagined, sleek and black in its plated carapace, vestigial eyes buried at either side of its pointed head, its mouth little more than a ridged wound; or if it was merely his mind's way of picturing the corrosion within himself, the foulness that had always been intrinsic to him. If the worm was present within him then it was evil, and some part of the pleasure that he felt was shared, or even generated by that alien presence. But even if it did not exist, there was still evil inside Buddy, evil beyond the worst atrocities he had witnessed on his TV screen, and Buddy knew it.

~John Connolly
Nocturnes

Many people think they have an "evil side" even if it's not quite as evil as Buddy's. Do you have an image of your "evil side"? A devil on your shoulder so to speak?