Benjamin Geen was born in 1980 in England. He pursued a career in nursing and, in April 2003, qualified as a registered nurse. Geen was known among colleagues for his "thrill-seeking" temperament and desire to be at the center of emergency situations. He joined the army reserve and was described as someone who wanted to "walk towards the action and not away from it." 

Despite being assigned to the minor injuries unit at Horton General Hospital in Banbury, Oxfordshire, he would often leave his post without authority to work in the emergency department. Colleagues observed that whenever patients unexpectedly fell ill, Geen was often present, leading them to nickname him "Ben Allitt," referencing the infamous serial killer nurse Beverley Allitt.

Between December 2003 and February 2004, Horton General Hospital experienced an unusual spike in respiratory arrests among patients with minor ailments. Eighteen patients suffered unexplained respiratory failures while under Geen's care, two of whom—Anthony Bateman (65) and David Onley (75)—died in January 2004. Onley had been admitted for heart problems and diabetes complications; his condition improved until Geen took over his care, after which he suffered a fatal respiratory arrest. Another patient, David Nelson, was found to have midazolam in his system, a drug not prescribed by any medical staff but known to cause respiratory arrest when administered incorrectly. Investigations revealed that Geen had administered unauthorized, potentially lethal doses of drugs, including midazolam and the paralytic agent vecuronium, to patients, causing them to stop breathing.

Hospital staff grew suspicious as respiratory arrests occurred predominantly during Geen's shifts. Colleagues noted that he often anticipated medical emergencies before doctors arrived, suggesting prior knowledge of the patients' sudden deteriorations. Some staff testified that Geen appeared "elated" during these emergencies and even boasted, "There is always a resuscitation when I'm on duty." ​

On February 9, 2004, Geen was arrested upon arriving at the hospital. He was found carrying a syringe containing the paralytic drug vecuronium, despite hospital protocols strictly forbidding nurses from taking syringes or such drugs out of the hospital. As officers approached, Geen attempted to conceal the syringe by discharging its contents into his jacket pocket. Tests later revealed that the jacket contained traces of both vecuronium and midazolam, further implicating him in the unauthorized administration of these drugs.

During his trial at Oxford Crown Court, it was revealed that Geen purposely used insulin, sedatives, and muscle relaxants to trigger respiratory arrests because he enjoyed the "thrill" of resuscitating patients. Both the prosecution and defense acknowledged that he was frequently present during unexpected patient emergencies. On April 18, 2006, Geen was found guilty of two counts of murder and 15 counts of grievous bodily harm. He received 17 life sentences with a recommendation to serve at least 30 years before being considered for parole. Throughout the trial and subsequent appeals, Geen has maintained his innocence.