Poisoners

Ronald Clark O'Bryan (October 19, 1944 – March 31, 1984), nicknamed The Candy Man and The Man Who Killed Halloween, was an American man convicted of killing his eight-year-old son Timothy (April 5, 1966 – October 31, 1974) on Halloween 1974 with a potassium cyanide-laced Pixy Stix that was ostensibly collected during a trick or treat outing. O'Bryan poisoned his son in order to claim life insurance money to ease his own financial troubles, as he was $100,000 in debt. O'Bryan also distributed poisoned candy to his daughter and three other children in an attempt to cover up his crime; however, neither his daughter nor the other children ate the poisoned candy. He was convicted of capital murder in June 1975 and sentenced to death. He was executed by lethal injection in March 1984.

Hélène Jégado (1803 – 26 February 1852) was a French domestic servant and serial killer. She is believed to have murdered as many as 36 people with arsenic over a period of 18 years. After an initial period of activity, between 1833 and 1841, she seems to have stopped for nearly ten years before a final spree in 1851.

Seweryn Antonowicz Kłosowski (14 December 1865 – 7 April 1903), better known under his pseudonym George Chapman, was a Victorian era Polish serial killer known as the Borough Poisoner.
Born in Congress Poland, Chapman moved to England as an adult, where he committed his crimes. He was convicted and executed after poisoning three women, but is remembered today mostly because some contemporary police officers suspected him of being the notorious serial killer "Jack the Ripper".

Christine Malèvre (born 10 January 1970) is a former nurse who was arrested in 1998 on suspicion of having killed as many as 30 patients. She confessed to some of the murders, but claimed she had done so at the request of the patients, who were all terminally ill. France, however, does not recognize a right to die, and Malèvre eventually recanted most of her confessions. The families of several of her victims strongly denied that their relatives had expressed any will to die, much less asked Malèvre to kill them.

Colin Campbell Norris (born 12 February 1976) is a Scottish serial killer and former nurse convicted for the murder of four elderly patients and attempted to murder another in two hospitals in Leeds, England in 2002.
Norris, who self-admittedly disliked elderly patients and had previously stolen hospital drugs, was the only person on duty when all the five patients inexplicably fell into sudden hypoglycaemic comas, despite the non-diabetic women only being in minor injury wards with merely broken hips. Suspicions were raised when Norris predicted that healthy Ethel Hall would die at 5:15 am one night, which is when she fell into a catastrophic arrest, and tests revealed that she had been injected with an extremely high level of man-made insulin. Insulin was missing from the hospital fridge and Norris had last accessed it, only half an hour before Hall fell unconscious.
Subsequent investigations would find that the unnatural hypoglycaemic attacks followed him when he was transferred to a second hospital, and hospital records revealed that only he could not be eliminated as a suspect. Detectives believed that Norris was responsible for up to six other suspicious deaths where only he was always present, but a lack of post mortem evidence and other factors meant that investigators and the Crown Prosecution Service could not pursue convictions for these deaths. The murder inquiry was led by Chris Gregg and the investigation was praised for its thoroughness.
Doubts were later raised about his conviction by, among others, Professor Vincent Marks, an expert on insulin poisoning, who concluded from his own studies that there was a 1 in 10 chance that each patient's arrest could have happened naturally. However, others have pointed out that C-peptides are produced in hypoglycaemic attacks caused by insulin produced naturally in the body, and these were not detected in any of the blood tests of the victims, indicating that the insulin had been introduced to their bodies externally and artificially. Norris lost an appeal against his conviction in 2009. In February 2021 the Criminal Cases Review Commission referred the case back to the Court of Appeal.
Norris is believed to have been inspired by Jessie McTavish, a fellow Scottish nurse who was convicted of murdering a patient with insulin in 1974 before having her conviction quashed in 1975. The incident had happened at Ruchill Hospital in Glasgow, less than a mile from where Norris grew up. Shortly before he qualified as a nurse he had learned about McTavish.

Kristen Heather Gilbert (née Strickland; born November 13, 1967) is an American serial killer and former nurse who was convicted of four murders and two attempted murders of patients admitted to the Veterans Affairs Medical Center (VAMC) in Northampton, Massachusetts. She induced cardiac arrest in patients by injecting their intravenous therapy bags with lethal doses of epinephrine, commonly known as adrenaline, which is an untraceable heart stimulant. She would then respond to the coded emergency, often resuscitating the patients herself. Prosecutors said Gilbert was on duty for about half of the 350 deaths that occurred at the hospital from when she started working there in 1989, and that the odds of this merely being a coincidence was 1 in 100 million. However, her only confirmed victims were Stanley Jagodowski, Henry Hudon, Kenneth Cutting, and Edward Skwira.

Marcel André Henri Félix Petiot (17 January 1897 – 25 May 1946) was a French medical doctor and serial killer. He was convicted of multiple murders after the discovery of the remains of 23 people in the basement of his home in Paris during World War II. He is suspected of the murder of about 60 to 200 victims during his lifetime, although the true number remains unknown.Despite showing early signs of mental illness and criminal behaviour, Petiot served in the First World War, graduated from an accelerated medical program, and began a dubious medical career that included performing abortions and supplying narcotics. His political career was marked by scandal, theft, and corruption. During the Second World War, Petiot operated a fraudulent escape network, offering safe passage to those wanted by the Germans for a fee, only to murder them, steal their valuables, and dispose of their bodies. In total, he was suspected of around 60 murders, but the remains of only 23 victims were found in the basement of his Paris home. Captured in 1944, Petiot claimed to be a Resistance hero who killed only the enemies of France. He was convicted of 26 counts of murder and was executed by guillotine in 1946. His life and heinous crimes have been depicted in film and comic books.