I think it's a power fantasy specific to women. A villainess character is powerful in the social sphere both personally and in status. She's also freed from certain societal expectations associated with "proper" women because she is assumed to be "Bad". Although in a lot of Villainess stories the characters end up being nice characters anyways, but they still have the excuse to be more forward and brash then female characters might usually be in Japanese works.
Subcategories
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Ascendance of a Bookworm
If there aren't any books, I'll just have to $@#! up society until I get them!
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How a Realist Hero Rebuilt the Kingdom
Step into my office and let me tell you about crop rotation...
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My Next Life as a Villainess: All Routes Lead to Doom!
Can't die by the hero's hand if all the heroes love you!