11. Genghis Khan
As punishment, Genghis Khan would kill your family, your family’s family, and perhaps even your family’s family’s family if you betrayed him. And that’s not the scariest thing about him.
The scariest thing about Khan was that he was incredibly smart. He managed to win countless battles, a lot of which he won while his forces were outnumbered. Trickery allowed him to do so; one of his most famous tricks was putting straw-men on horses to trick his enemies into thinking he had more forces.
Khan was incredibly ruthless. For example, he murdered his half-brother when he refused to share food. All in all, Genghis Khan killed nearly 10% of the world’s population at the time.