The Evil Leaper (Quantum Leap)
If there was ever an "evil female" character that had excuses made for her, it was the Evil Leaper. She wasn't evil; she was just forced to do evil things by her masters. Once Sam gave her chance to join the "good side," she immediately jumped at it.
The problem is, there are, in fact, very few purely villianous female characters. They almost invariably have some excuse for their actions (they're really good, just misguided) (they're being forced to be evil).
There are some, however....
1.) The White Witch (The Chronicles of Narnia)
2.) Bellatrix Lestrange (Harry Potter)
3.) Maleficent (Sleeping Beauty)