Osama bin Laden
As the founder and head of Al-Qaeda, Osama bin Laden is responsible for the worst terrorist attack in the United States on 9/11 with more than 3,000 people killed and another 6,000 injured.
Like Osama bin Laden, Saddam Hussein is a name synonymous with relatively recent terrorism. Not only did he kill 2,000,000 of his own citizens during his run as Iraq's president, he was also known for recording these executions and watching them at home for his own amusement.
Another member of the Nazi regime who carried out spine-chilling death sentences to 6-7 million Russians, Jewish Poles, and communists. He was later discovered to have also commissioned furniture and books to be made from the bones and skin of his victims.
After coming into power during the French Revolution, Maximilien Robespierre killed people indiscriminately - including his closest friends and family through horrific executions.