2. The Mongols
From 1206 to 1368 the Mongols slaughtered over 11% of the world’s population, which is around 40 million people. Their goal was to conquer others and build the biggest empire world has ever seen under their leader Genghis Khan. In that process, a lot of innocent civilians, including pregnant women died. Their worst method of killing was catapulting corpses over city walls in order to infect entire populations.