Since time began, storytellers have liked adding twists into their stories, to keep the tales from seeming predictable, stale or generic. This is especially (though not exclusively) true of modern storytellers. One such twist is to take a heroic character, a protagonist or ally of a protagonist and corrupt them, turning them to the side of the antagonists - making them Evil. This twist is especially popular in fantasy and Sci-Fi, where Good and Evil are usually clearly defined and where the most heroic are those who are most likely to be attracted to the "easy way out" offered by Evil.
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