Britain's serial-killer monarch is estimated to have executed somewhere between 57,000-70,000 people during his 36-year-reign, including friends, advisors, clergy, old people, children, and two of his wives.

Caligula earned a brutal reputation despite ruling for only four years, from AD 37 to 41. His reign began with policies such as cutting taxes and pardoning exiles, and historians debate whether syphilis, epilepsy or mental illness changed him.

King John I is known as the Medieval Monster and inspired the fictitious villain of Robin Hood legends. Even before his ascension to the throne, he tried to claim that Richard the "Lionheart" died during the crusades so he could rule instead.

Tsar Ivan IV Vasilyevich was called the "Terrible" back when it meant awe-inspiring, powerfu' and formidable. By the end of his reign, most of his subjects probably felt that the modern interpretation of the word was more appropriate.

Emperor Nero is known for executing Christians for fun. Nero was 17 when he ascended to power in AD 54 following the death of his step-father, Claudius, Fourteen bloody years later, Nero died at the climax of a rebellion against his rule.

King Herod the Great is a biblical child-killer who didn't just murder other people's children but killed his own offspring too. He earned his moniker through terrible conquests and brutal victories.