Agrippina the Younger
(16-59 AD)

Her mother, Agrippina the Elder, was a pillar of virtue; Agrippina the Younger elected to take the opposite route. Only Roman woman to sleep with three Emperors to whom she was related: (1) was concubine to her brother Caligula (along with her two sisters), (2) married her uncle, Claudius, and (3) seduced her son, Nero. After she'd secured the throne for Nero by poisoning Claudius, she settled scores via assassination; on one occasion, she ordered the head of a female victim to be brought to her so that she could reach inside the head's mouth and confirm her kill by a telltale irregularity of the teeth. After enjoying sex with Nero for many years, she was stabbed to death by soldiers while he watched.

Sources: Dio, LXI & LXII; Suetonius, Twelve Caesars; Tacitus, Annals