Here's why and some options:
* "Villain" itself is gender-neutral: It's traditionally been used for male characters, but the word itself doesn't inherently specify gender.
* Context matters: The best term depends on the context. If you need to specifically highlight the character's gender, you could use terms like:
* Vile woman/woman of evil
* Female antagonist/female adversary
* She-villain (though this is somewhat informal and can sound childish)
* Wicked witch (if appropriate for the character's specific traits)
* Focus on the character's actions: Instead of seeking a specific feminine equivalent, consider describing the character's actions and motives, which will be more impactful than simply using a gendered term.
Ultimately, the best way to refer to a female villain depends on the specific character and the context of your writing.