A follow up, Ghengis's campaign put multiple cities to the sword, executing everyone not of direct use to the campaign. Death tolls are hard to estimate, but you're looking at anywhere from on the order of tens of thousands to a million people executed at the same time—over the course of hours. Famously, the Tigris ran black and red from ink and blood after the sacking of Baghdad (the ink came from the Grand Library of Baghdad), and the destruction of the Khwarezmia dynasty was particularly bloody.
Who knows if we'll ever get accurate numbers, but it's one of the most gruesome period of human history. The institutional hate towards Ghengis only disappeared in the 20th century.
