Listed here are some of the leading mass murderers of the 20th and 21st Centuries as compiled on Wikipedia.  

This list is only confined to those who have been arrested, tried and convicted by traditional criminal courts.  Not
included are war criminals, such as Nazis, or those sought, arrested, tried or put to death under the auspices of
authorities empowered to deal with war crimes, genocide, etc. Wikipedia also has a link on that page for mass
murderers prior to the 20th Century.

The complete list of these mass murderers can be found here at
Luis Alfredo Garavito Cubillos    VICTIM COUNT:  172-400 Plus

This gem of a Columbian citizen leads the world list in victims.

In 1999, he admitted to the rape and murder of 147 young boys.] The number of his victims, based on the
locations of skeletons listed on maps that Garavito drew in prison, could eventually exceed 300. He has
been described by local media as "the world's worst serial killer" because of the high number of victims.
Once captured, Garavito was subject to the maximum penalty available in Colombia, which was 30 years.
However, as he confessed the crimes and helped authorities locate bodies, Colombian law allowed him to
apply for special benefits, including a reduction of his sentence to 22 years and possibly an even earlier
release for further cooperation and good behavior. Colombian law has since increased the maximum
penalty to 60 years in prison.
Yang Xinhai                                        VICTIM COUNT:  67

Yang's killings took place between 1999 and 2003 in the provinces of Anhui, Hebei, Henan and Shandong. At night, he
would enter his victims' homes, and kill all of the occupants—mainly farmers—with axes, hammers and shovels,
sometimes killing entire families. Each time he wore new clothes and large shoes.  Shortly after he was arrested, Yang
confessed to 65 murders, 23 rapes and five attacks causing serious injury: 49 murders, 17 rapes and five attacks in
Henan; eight murders and three rapes in Hebei; six murders and two rapes in Anhui; and two murders and one rape in
Shandong. Police also matched his DNA with that found at several crime scenes. Later it was discovered that Xinhai
contracted HIV from one of his victims. On February 1, 2004, Yang was found guilty of 67 murders and 23 rapes, and
sentenced to death in Luohe City Intermediate People's Court, Henan. At the time of his sentencing, official Chinese
media believed he had carried out China's longest and grisliest killing spree. Yang was executed on February 14, 2004
by shooting.
Andrei Romanovich Chikatilo                        VICTIM COUNT:  56

Chikatilo was a Soviet serial killer, nicknamed The Butcher of Rostov, The Red Ripper, and The Rostov Ripper, who
committed the sexual assault, murder and mutilation of a minimum of 52 women and children between 1978 and 1990
in the Russian SFSR. Chikatilo confessed to a total of 56 murders and was tried for 53 of these killings in April 1992.
He was convicted and sentenced to death for 52 of these murders in October 1992 and subsequently executed in
February 1994.  Chikatilo was known by such titles as The Rostov Ripper and The Butcher of Rostov because the
majority of his murders were committed in the Rostov Oblast of the Russian SFSR.  On 14 February 1994, Chikatilo
was taken from his death row cell to a soundproofed room in Novocherkassk prison and executed with a single gunshot
behind the right ear.
Anatoly Yuriyovych Onoprienko                        VICTIM COUNT:  52 Plus

Here we have yet another "winner" from the former Soviet Union.  He is a Ukrainian serial killer and mass murderer.
He is also known by the nicknames "The Beast of Ukraine", "The Terminator", and "Citizen O". After police arrested
the 37-year-old former forestry student on April 16, 1996, Onoprienko confessed to killing 52 people.  When finally
arrested by police, Onoprienko was found to be in possession of a total of 122 items, including a sawed-off Tos-34
hunting rifle, a number of other weapons, which matched the murder weapons used in several of the killings, and a
number of items which had been removed from murder victims. While in custody, he eventually confessed to eight
killings between 1989 to 1995. At first, he denied other charges, but ultimately confessed to the killing of 52 innocent
victims over a six-year period. While in custody, he claimed that he killed in response to commands he was given by
inner voices.  He is serving a life sentence.
Alexander Yuryevich "Sasha" Pichushkin                VICTIM COUNT:  60 Plus

Whoa!  What is this?  A trifecta of evil from former Communist Russia.  This guy just didn't like to make long trips
from home to find his victims.  also known as The Chessboard Killer and The Bitsa Park Maniac, is a Russian serial
killer. He is believed to have killed at least 49 people and up to 61–63 people in southwest Moscow's Bitsa Park, where
several of the victims' bodies were found.  He was arrested on 15 June 2006, and convicted on 24 October 2007 of 49
murders and three attempted murders.  He asked a Russian court to add an additional 11 victims to his body count,
bringing his claimed death toll to 60 and 3 surviving victims.  During the trial, he was housed in a glass cage.  It took
Judge Vladimir Usov an hour to read the verdict: life in prison[13] with the first 15 years to be spent in solitary
Gary Ridgway                                                VICTIM COUNT:  90 Plus

The U.S.A. rings in with this painter born in Utah.  Ridgway was a truck painter who confessed to killing 71 women.
The United States' most prolific serial killer; also known as The Green River Killer. He almost exclusively targeted
escorts from Seattle. Once there, he would proposition a prostitute, drive to the banks of the Green River, and strangle
her to death. After murdering his victim he would dump her body near the nearby water. Suspected of killing over 90
victims, confessed to 71, convicted of 49.  On November 30, 2001, as he was leaving the Renton, Washington Kenworth
Truck factory where he worked, he was arrested for the murders of four women whose cases were linked to him through
DNA evidence.[2] As part of a plea bargain wherein he agreed to disclose the whereabouts of still-missing women, he
was spared the death penalty and received a sentence of life imprisonment without parole.
Theodore Robert "Ted" Bundy                              VICTIM COUNT:  36 Plus

Bundy was an American serial killer, rapist, kidnapper, and necrophile who assaulted and murdered numerous young
women and girls during the 1970s and possibly earlier. After more than a decade of denials, he confessed shortly before
his execution to 30 homicides committed in seven states between 1974 and 1978; the true total remains unknown, and
could be much higher.  Initially incarcerated in Utah in 1975 for aggravated kidnapping and attempted criminal
assault, Bundy became a suspect in a progressively longer list of unsolved homicides in multiple states. Facing murder
charges in Colorado, he engineered two dramatic escapes and committed multiple additional assaults, including three
murders, before his ultimate recapture in Florida in 1978. He received three death sentences in two separate trials for the
Florida homicides.  Ted Bundy died in the electric chair at Raiford Prison in Starke, Florida, on January 24, 1989
John Wayne Gacy, Jr.                                                 VICTIM COUNT:  34

Gacy was known as the Killer Clown, who was convicted of the sexual assault and murder of a minimum of 33 teenage
boys and young men in a series of killings committed between 1972 and 1978 in Chicago, Illinois.  All of Gacy's known
murders were committed inside his Norwood Park home; his victims would typically be lured to this address by force or
deception and all but one victim was murdered by either asphyxiation or strangulation with a tourniquet (his first
victim was stabbed to death). Gacy buried 26 of his victims in the crawl space of his home; three further victims were
buried elsewhere on his property, while the bodies of his last four known victims were discarded in the Des Plaines
River.  Convicted of 33 murders, Gacy was sentenced to death for 12 of these killings on March 13, 1980. He spent a
total of 14 years on death row before he was executed by lethal injection at Stateville Correctional Center on May 10,
1994. Gacy became known as the "Killer Clown" due to his charitable services at fundraising events, parades and
children's parties where he would dress as "Pogo the Clown", a character he devised himself.