Stephan Letter was born on September 17, 1978, in Herdecke, a small town in North Rhine-Westphalia, Germany. He eventually became a nurse and took a job at a hospital that specialized in treating elderly patients. He worked at this hospital from January 2003 until July 2004. 

During his time working as a nurse from January 2003 to July 2004, Stephan Letter was linked to a disturbing number of patient deaths at the hospital in Sonthofen, Bavaria. A total of over 80 deaths took place on his shifts, raising suspicions among hospital officials. This led to investigations where officials exhumed the bodies of more than 40 patients. However, 38 other bodies had been cremated, making it impossible to examine them.

Investigators found that large amounts of drugs, including the paralytic drug Lysthenon, were missing from the hospital. Unsealed medication vials were later discovered in Letter's apartment, further complicating the case against him.

In February 2006, Letter faced a trial concerning the deaths of 29 patients. He was charged with 16 counts of murder, 12 counts of manslaughter, and one count of killing on request. The patients he was accused of killing were mostly older than 75, but their ages ranged from 40 to 94.

One notable incident during the trial included an inappropriate injection administered to a 22-year-old soldier, who had minor injuries from a fall. She lost consciousness due to the injection but eventually recovered. During the trial, Letter admitted to some of the killings but claimed he acted out of sympathy to relieve the suffering of patients.

However, the prosecution argued that Letter was not assigned to care for some of the patients he killed. Some patients were stable and were supposed to be released from the hospital soon. In November 2006, he was found guilty of the murders and received a life sentence in prison. He is currently held at a facility in Straubing. His actions have been described as the worst killing spree in Germany since the Second World War.