The first known serial killer was actually a woman, known as Locusta, a professional poisoner who lived in Rome during the first century A.D.
In 54, she killed the Emperor Claudius with a poisoned dish of mushrooms, and the following year she was convicted of a separate poisoning. Hearing of this, Nero rescued her from execution — so she could poison Britannicus for him.
They made a good partnership, Nero guaranteeing her safety during his lifetime, but when he died the Romans took an awful revenge. According to legend, Locusta was publicly raped by a specially trained giraffe, then torn apart by wild animals. Talk about cruel and unusual.