The 25 most ruthless leaders of all time

Reign: A.D. 434-453

After killing his brother, Attila became the leader of the Hunnic Empire, centered in present-day Hungary, and ended up becoming one of the most feared assailants of the Roman Empire.

He expanded the Hunnic Empire to present-day Germany, Russia, Ukraine, and the Balkans. He also invaded Gaul with the intention of conquering it, though he was defeated at the Battle of Catalaunian Plains.

"There, where I have passed, the grass will never grow gain," he reportedly remarked on his reign.

Source: Britannica, Biography

Reign: A.D. 37-41

Caligula was quite popular at first because he freed citizens who were unjustly imprisoned and got rid of a stiff sales tax. But then he became ill, and he was never quite the same again.

He eliminated political rivals (forcing their parents to watch the execution), and declared himself a living god. According to Roman historian Suetonius, Caligula had sex with his sisters and sold their services to other men, raped and killed people, and made his horse a priest.

He was eventually attacked by a group of guardsman and stabbed 30 times.

Source: Biography.com, BBC, "Atlas of History's Greatest Heroes and Villains" by Howard Watson.

Reign: 247-210 B.C.

Qin, also called Qin Shihuangdi, united China in 221 B.C. and ruled as the first emperor of the Qin dynasty. He was known to order the killing of scholars whose ideas he disagreed with and the burning of "critical" books.

During his reign, he ordered the construction of a great wall (roughly speaking, the prequel to the modern Great Wall of China), and an enormous mausoleum featuring more than 6,000 life-size terra-cotta soldier figures. Large numbers of conscripts working on the wall died, and those working on the mausoleum were killed to preserve the secrecy of the tomb.

"Every time he captured people from another country, he castrated them in order to mark them and made them into slaves," Hong Kong University's Xun Zhou told the BBC.

Source: British Museum, Britannica, History, BBC