The True Cause

Lycanthropy is a supernatural curse that causes the victim's spiritual essence to become mingled with the wild essence of nature, symbolized by the wolf. The lycanthrope in effect has two souls, or one split soul. These two essences constantly battle for control within the victim. When the wild wolf-essence triumphs, the werewolf change occurs. This may explain why werewolves hunt humans so often; the wolf-essence desires to destroy the human side and triumph over humanity, and does so symbolically by brutally slaying humans.

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PLANESWALKER'S GUIDE TO INNISTRAD: KESSIG AND WEREWOLVES