Basically, the Mongols wiped out everyone from urban areas, unless they submitted to the Mongols. They had the choice between joining or dieing. In addition to these tactics, their speed of expansion was facilitated by military hardiness, military skill, meritocracy, and discipline. They managed to create the largest contiguous land empire in the known human history and to wipe out more then 50 million people!
The UN list of genocides (16 at this moment (August 2014)) with almost 30 million people killed is not complete. I need to note that the large (by numbers) genocides are performed by the Mongols (in the year 1200 and after) where 50-70 million people died and Stalin with hisgenocide, killing 43-60 million people!