Because the personal computer had not yet been invented, records keeping was not up to today's standards, but according to the pundits of the time, it is possible that this title belongs to the Mongols.

Beside the Chinese, the Japanese, the Indians, various Europeans, they conquered most of Islam, heaping mountains of skulls in the process. Given their trend to exagerate beyond all understanding, historians of the time still say the numbers ran into tens of millions. It is conceivable. When they encountered a community that resisted them, they were known to put whole cities to the sword--literally.

The world population might have been smaller then, so the gruesome numbers might not have been close to the 20-30 million figures we're tossing around here but chronicals of the time were in agreement that these were some cru-well dudes. If they did not kill that many, it was because their sword arms got weary.

De mortuis nihil nisi bene.