1. Mid 1400s: Vlad the Impaler had 30,000 of his own people impaled slice and erected around cities in Transylvania to scare off the Turks. 2. Europeans folding smallpox scabs into blankets so that entire communities of aboriginals would die without wasting bullets so that their land could be seized. 3. Forcefully removing First Nations children from their families and placing them in residential homes with the agenda to dismantal their cultural knowledge. All this in order to manage their activities better. There is more . . .
The attacks on the twin towers and the pentagon had the largest death tolls and affected the most countries in a single incident. Killing 2,976 people. But if the truth be told the KKK killed more people between the years 1865–1877. It was not one big attack but it was a constant terrorist threat.
Several terrorist campaigns come to mind that exceed 9/11 in terms of social effect and number killed. 1. The Communist Chinese "reforms" from 1955-1989 killed or starved over 39 million (average estimate). 2. Stalin killed over 20 million in his purges. 3. France had the Great Terror: the Reign of Terror (5 September 1793 to 28 July 1794) also known simply as The Terror (French: la Terreur), was a period of violence that occurred after the onset of the French Revolution, incited by conflict between rival political factions, the Girondins and the Jacobins, and marked by mass executions of "enemies of the revolution." The death toll ranged in the tens of thousands, with 16,594 executed by guillotine (2,639 in Paris), and another 25,000 in summary executions across France. 4. Tamerlane (c 1360) captured the city of Herat in 1383 and went on to take much of Persia as well. He ruled fairly according to many reports but when confronted by any kind of revolt or resistance was known to slaughter entire cities to a man. In the city of Isfahan, Tamerlane built pyramids made from thousands of skulls. He did this in response to a revolt in which several of his tax collectors were killed. When Tamerlane began his invasion of India, he became truly brutal. He took the city of Delhi in 1398 and when he did, put 100,000 people to death. Totals suggest that he may have killed as many as fifteen to twenty million people during the course of his military career.