We tend to think of men as being the biggest assholes in history.
But as an advocate of gender equality, I feel that it is necessary to point out that evil isn’t sexist. The woman I have picked is often ignored when discussing serial killers.
It is not the fact that she killed an estimated 650 people that really demonstrates her evilness. It’s how she did it. She tortured and killed her victims with impressive creativity. Whether it was forcing her victims to eat their own cooked flesh or slathering them in honey for the insects to eat alive, she always found ways to make her killing interesting. In the case of killing little girls (her primary target in the beginning), she would bathe in their blood as she thought it would preserve their youth.
My 2nd pick is the serial killer was known as The Blood Countess, Elizabeth Báthory de Ecsed.