From what I've read the biggest mass murderer was Ghengis Khan.
He knew that people often HID OUT and HID their valuables and daughters, so Genghis would RETURN to the site of his last attack, pillage, burn, rape and kill to FINISH the job.
He was one mean S.O.B. in a human history of mean S.O.B.s
The Mongols, in about a century, managed to kill about 10% of the world’s population.
No other group has ever come close to that record.
The Nazis, for example, killed perhaps 20M people, and the Commies perhaps 125M, out of a population of perhaps 2.5B.
That’s 0.8% for the Nazis and 5% for the commies.