Murderess
/ˈmɜːrdəres/
Definition 1 of 1
Pro Tip 1/3Gender-Specific
Used only for female killers; 'murderer' is gender-neutral or used for males. 
The novel featured a cunning murderess.
Pro Tip 2/3Legal Context
Common in legal situations describing a woman who has unlawfully taken a life. 
The murderess was sentenced to life in prison.
Pro Tip 3/3Historical Usage
It's an old-fashioned term, less common in modern language, and may carry historical tones. 
In the 19th century, she would have been called a murderess.