Sort of a ghoulish topic, but something people need to know about...
What does it mean to be a mass murderer? How many people do you have to kill to get on the list, and what does it take to get up towards the top of the list?
The term is overused to a large extent. There are only about a half dozen or so people in the upper ranks of mass murderers, and the gulf between that half dozen or so and the people on the next rung down is so vast that it almost can't be viewed as the same thing.
Working from the bottom upwards, you'd start off with ordinary criminals who simply rose a little above the norm in the course of the criminal business, or who developed a fondness for killing people and began to do it as a hobby rather than as part of their business. One of these guys might end up killing a dozen to a few hundred people, enough for garish headlines, but several orders of magnitude from being worthy of the history books. These guys would be the bottom tier.
For the tiers above the bottom one, being a criminal would not suffice; you need to be a politician. The next tier upwards would consist of every tribal chief, king, or military commander whose numbers ever got up into the thousands or tens of thousands.
The next tier would consist of people who, both in modern and ancient times, managed to kill somewhere between a few hundred thousand and a few million people, but not tens of millions. This tier would include Atilla the Hun, Genseric the Vandal, Idi Amin, Pol Pot, and all such similar types.
Atilla the Hun
At some point in such a study, you'd get to your really serious kinds of mass murderers. The first person in history who anybody would associate with the first tier of mass murderers was Chengis Khan.
"...Also, Genghis Khan's waging of war was characterized by wholesale destruction on unprecedented scale and radically changed the demographic situation in Asia. According to the works of Iranian historian Rashid-ad-Din Fadl Allah, Mongols massacred over 700,000 people in Merv and more than a million in Nishapur. China suffered a drastic decline in population as a direct result of the Khan: before the Mongol invasion, China had about 100 million inhabitants; after the complete conquest, 1279, the census in 1300 showed it to have roughly 60 million people. This does not, of course, mean that Genghis Khan's men were directly responsible for the deaths of 40 million people, but it does give a sense of the ferocity of the onslaught."
The large, settled areas of the world at the time of Chengis Khan included North China (Kin realm), Southern China (Sung), Turkistan/Khwarism, India, and Europe/Russia and of those only India escaped the attentions of the Mongols, who had learned to wage wars over degrees of lattitude and longitude while most of the world's peoples never ventured more than twenty miles from the places of their births.
Other than for warfare in China however, after the invasions of Europe around 1240 and the levant around 1256, Mongols embarked upon no further foreign wars which involved killing vast numbers of people, and settled down into a lifestyle of trade and occasional raids for plunder.
That went on for several generations until a man named Timur, or Tamerlane arose in the mid to late 1300s to try to reestablish a global empire along lines similar to those established by Chengis Khan.
Some view Tamerlane as being another first tier mass murderer and for sure he did establish dominion over an area which included parts of India, Russia, and the near East. Nonetheless he never got into China and for that reason his numbers could not have approached those of the Mongols proper. Tamerlane is mainly remembered for terror tactics, making pyramids of the skulls of opponents and the like.
Between the times of Chengis Khan and Tamerlane, and of the modern age, there are two episodes which anybody might figure belonged in the same league or close to it; one would be the inquisition, and the other the Thirty Years War.
From most of what I read, the inquisition has been overrated in terms of the sheer numbers of people who are commonly supposed to have fallen victim to it. Some of this is undoubtedly due to the fact of the inquisition having reached its zenith in southern Europe, chiefly Spain, and much of its history being written by the English who were the chief adversaries of Spain at the time. Nobody ever looks terribly good in the other guy's history books.
The thirty years war in the 1600s turned Germany into a pigpen from end to end and resulted in the deaths of millions of people, but not tens of millions. The one military chieftain who stands out in that war was King Gustavus Adolphus of Sweden and for that reason, some tend to lump him into the first tier of mass murderers along with the Subudais and Tamerlanes. Nonetheless, the causes of the Thirty Years War were so numerous and convoluted, that it's hard to give any one person credit for all of the grief it caused.
Including Gustavus Adolphus in the ranks with Tamerlane or Chengis Khan would therefore be a sort of a stretch.
Gustavus Adolphus of Sweden
Some might also want to rank Suleiman of Turkey into that same sort of category and there is no doubt he did his share of killing; nonetheless anybody who was decisevely defeated by the Poles does not belong on any list of conquerors or first or second tier mass murderers.
After the Thirty Years War, there was a long interlude without any killing on the same scale. The wars of the late 1600s and the 1700s were organized gangfights pitting royal houses against eachother, followed by the wars of Napoleon which did not kill vast numbers of people, and then most of a century without any major wars. WW-I killed large numbers of people but there was no one person you could blame for it, and then you arrive at the modern age and the three guys who resurrected the kinds of numbers which Chengis Khan used to put up.
Hitler, Stalin, and Mao Tse Tung
Mao Tse Tung
I would assume everybody is pretty much familiar with modern history and with the claims to fame (in terms of mass murder) of each of the three gentlemen pictured above. Now, if you didn't know better, you might think that that was pretty much the end of the story, and that somewhere between Chengis Khan and whoever is tops amongst the three guys pictured above, that had to be the all time record for mass murder, somewhere around 20 - 40 million people at the outside and the honest truth is that Chengis Khan in his own lifetime was probably at no more than 10 - 20 million, the big numbers for the Mongol empire coming after his death.
Nonetheless, anybody who had it figured that way would be wrong. There is a guy, in fact, who puts everybody mentioned here so far absolutely to shame, and who makes Chengis Khan, Tamerlane, Hitler, Stalin, Mao, Suleiman, Gustavus Adolphus and all the rest look absolutely like pikers.
William Ruckelshaus, a yuppy beaurocrat serving as first director of the Environmental Protection Agency (EPA) of the United States Government, by the single act of banning the super pesticide DDT, has killed around 90 (ninety) million people to date.
Virtually all of what most of us have read about DDT turns out to be bunk, and junk science. There were guys who fell into vats of it in the 50s who are still walking around, and there is no real evidence of it being harmful to birds. By the mid 50s, DDT had nearly wiped malaria, polio, and a number of other insect-borne plagues off of this planet; by a single misguided beaurocratic act, Ruckelshaus undid all of that and plunged much of the world back into a dark age of disease and grief.
Some might claim that Ruckelshaus was some sort of a racist intent upon ridding the planet of the kinds of people who live in malaria-infested areas. Much more likely, he was a typical beaurocrat obeying the primary dictum of all such people to cover his own ass, and heeding the ruckus and commotion caused by leftwing pseudoscientists and ideologues like Rachel Carson, whose book "Silent Spring" produced much of the disinformation we read about DDT.
What became of Rachel Carson? She'd probably down there screwing one of the guys mentioned on this page, in some sort of a boudoire which is kept at 1500 degrees F. I mean, for people with the deaths of 90 million people on their hands, airconditioning can become a problem...