it depends on your definition.
the question itself is too broad. for instance, 'most killed by a single mass murderer'? 'most killed in one day'? 'most killed by a regime'?
so let's answer each of these questions and it should give you the answer (and then some) you seek. the other question is, are you sure you're asking about 'mass murder' or 'serial killings'? just a quick run down for those that might not know the distinction...
(all of the following types of crimes are usually carried out by solitary individuals. there have been examples in all categories in which two or more perpetrators have acted together, but those are not covered here except where noted.)
Mass Murderers and Serial Killers:
a serial killer is someone who commits three or more murders over an extended period of time with cooling-off periods in between. a mass murderer, on the other hand, is an individual or group who commit multiple murders in a single event and in one location (or in the case of 9/11 in multiple locations). so by definition, any act of 'mass murder' that is not commited in a single day in a single act would be considered (a) serial killer(s), (but by my definition still mass murder(ers) as in the Nazis and the Holocaust).
let's start with the most killed by a regime;
Communists and Stalin: It is calculated that the Communist regime, 1917-1987, murdered about 62 million people, around 55 million of them citizens.
but, the number 1 in the shortest amount of time is;
Mao Tse Tung: During the years 1958 and 1959, China experienced what was hailed by Marxists as the "Great Leap Forward." During this time, Mao Tse Tung's regime was directly responsible for between 30 and 40 million deaths.
By comparison, the Holocaust saw 9-11 million people dead a result of Hitler and the Nazis.
most killed in a day;
The Atomic Bomb on Hiroshima killed 80,000 which is probably the highest civilian death toll in one day
Unsure what the greatest military deaths in one day would be.
most killed by one person;
Edit by Chris: Before all these next entires, there is one prime killer who beats almost all records.
Heinrich Severloh fired over 12.000 rounds and is said to have killed between 2000-2400 American GIs on Omaha beach with MG's and rifles.
Also, even then, before the next accounts of mass murderes we have Mihail Ilyich Surkov who sniped and killed around 700 people during WWII. And then, the many many many runner ups who also killed hundreds of people. Mainly Russian snipers, a few German, and a single finnish sniper who killed around 500 people.
A 16th century Hungarian countess named Elizebeth Bathory (August 7, 1560 - August 24, 1614) is said to have killed over 600 young women. She was convicted of only 80.
Harold Shipman an English general practitioner (January 14, 1946 - January 13, 2004) was convicted of killing only 15 but it was suggested he had actually killed over 215.
Henry Lee Lucas (August 23, 1936 - March 13, 2001) was an American criminal, convicted of murder and once listed as America's most prolific serial killer. Ultimately, Lucas was convicted of eleven homicides. He confessed to involvement in killing about 600 but the number is said to be closer to 350. (Strangely his is the only death sentence to be commuted to life in prison by then governer George W. Bush.)
most by 1 person in 1 day in the U.S.;
Timothy McVeigh (April 23, 1968 - June 11, 2001) with the Oklahoma City bombing claimed 168 lives. It is the deadliest incident of domestic terrorism in U.S. history.
In 1927, school board member, Andrew Kehoe (Feburary 1, 1872 - May 18, 1927), upset by a property tax that had been levied to fund the construction of the school building, killed 45 people (including himself) and injured an additional 58 in a bombing at the Bath Consolidated School. This is still the largest school massacre in United States history.
there are also 'spree killers':
A spree killer commits multiple murders in different locations over a period of time that may vary from a few hours to several days. Unlike serial killers, however, the spree killer does not take a long break or resume everyday life between killings.
Woo "Power" Bum-Kon (February 24, 1955-April 28, 1982) was a Korean police officer who carried out the worst incident of spree killing in known history, killing 58 (including himself) over 2 days time.
for a list of other serial killers go to:
http://en.wikipedia.org/wiki/List_of_serial_killers_by_number_of_victims
hope this helps!
Countess Erzsebet Bathory of Romania killed an estimated 600 young girls and bathed in the blood. Bathory died in 1614 after being found guilty of the murders, afterwhich she was confined to her own castle tower.
estimated that Gilles de Rais killed 140-800 people ( children, mainly )
Elizabeth Bathory killed more than 600.
Anatoly Onoprienko killed about 52 in the 1900's
Pedro Alonso Lopez killed over 300 people ...
Likely the most prolific serial killer of modern times was Pedro Alonso Lopez. He was a Colombian who is thought to have raped and killed over 300 young girls in the 1960s and 1970s.
Mao KILLED SEVENTY SEVEN MILLION HUMAN BEINGS AND STALIN KILLED FORTY THREE MILLION, ANY THING ELSE IS PALES IN COMPARISON
That was Dr. Harold Shipman of the UK with 218 proven victims and a probable 250+ He killed his patients mostly old women with diamorphine injections.
Green river killer
Classification of serial killer is that they have killed three or more people.
There was a rumor that Tommy Debarge's wife Duckie, was killed by Alton Coleman, a black serial killer. I have not found any document to that effect and the list of the Alton Coleman's victims did not include her. Being killed by a serial killer is supposed to make head lines.
Jeffrey Dahmer.
The Old Testament portrays Joshua as a serial killer who mercilessly killed all the inhabitants of every city he conquered.
I Killed My BFF - 2012 My Best Friend the Serial Killer 1-8 was released on: USA: April 2013
Cherie Ayers had been bludgeoned and stabbed repeatedly in the neck and killed on October 9, 1980 by Randall Brent "Randy" Woodsfield who is an American serial killer who was dubbed The I-5 Killer or The I-5 Bandit.
Andrew cucanon a serial killer killed shot johnny versace in front of his house
For some odd reasons some serial killers choose children as prey. Andrei Romanovich Chikatilo was a notorious Russian children serial killer who also killed women.
The most famous serial killer to me is Mary Bell, because she started in her crimes at 10 years of age.
Absolutely not. An alcoholic craves alcohol. A serial killer craves power. Some serial killers abuse alcohol and/or drugs but most do not.
I could find no serial killer from Stockton, CA. But an English serial killer was named The Stockwell Strangler. Kenneth Erskine killed 7 to 11 elderly women from April 9, 1986 to July 23,1986. That's a large number of victims for such a short period of time.
yes, she is a serial killer from springdale arkansas, she is in the MPIC program in archer learning center. she killed 18 people in washington county jdc. if you see her, call police immediately.