Niels Högel was born on December 30, 1976, in Wilhelmshaven, a coastal town in Lower Saxony, West Germany. His family included his father, who worked as a nurse at the Sankt-Willehad Hospital, and his mother, who was a paralegal. He had an older sister. Högel described his childhood as "sheltered" or "protected." He claimed that he was not exposed to violence while growing up. Additionally, his grandmother also worked as a nurse.

Högel completed his vocational training at the Sankt-Willehad Hospital in 1997. After finishing his training, he became a nurse and continued to work at the same hospital. In 2004, he got married, and later that year, his daughter was born. However, he struggled with issues related to alcohol and analgesic abuse throughout his life.

In a psychiatric evaluation that took place in 2019, experts described Högel as having a combined personality disorder. They noted traits of narcissism, dissocial behavior, and compulsiveness. He had a strong need for recognition, a desire for excitement, and showed a remarkable lack of empathy. He did not appear to feel guilt, shame, or remorse.

Niels Högel began his nursing career at the Oldenburg Clinic in 1999. He worked on the cardiac surgery intensive care unit, Ward 211. In August 2001, a meeting was held to discuss a sudden increase in patient resuscitations and deaths. During the meeting, it was noted that 58% of these incidents occurred while Högel was on duty. Following this meeting, Högel took three weeks off. During his absence, the number of patient deaths significantly decreased.

Due to growing concerns, Högel was moved to the anesthesiology ward in 2001. There, the head physician became suspicious of the frequent emergencies that arose when Högel was present. By September 2002, the head physician confronted Högel about patients in critical condition for no clear reason. It was suggested that he either resign or be transferred to a less critical position within the hospital.

In December 2002, Högel moved to the Delmenhorst Clinic. After he began working there, instances of emergencies and patient fatalities rose sharply. His colleagues started to distance themselves from him as suspicion grew. On June 22, 2005, Högel was caught manipulating a syringe pump to administer a medication improperly. This incident led to his arrest.

After his arrest, several coworkers spoke up about their concerns regarding Högel's involvement in numerous patient complications and unexplained deaths. An investigation revealed that the number of deaths at the Delmenhorst Clinic had doubled during Högel's employment. In 2006, he was initially sentenced to five years in prison for attempted voluntary manslaughter.

Further investigations led to more charges against Högel. In 2015, he confessed to killing patients by administering unauthorized injections. The details showed that he had injected patients with powerful drugs that caused life-threatening conditions. On February 28, 2015, Högel was sentenced to life in prison.

Subsequent investigations identified 200 additional suspicious deaths during his nursing career. By late 2017, authorities concluded that Högel was responsible for the deaths of at least 90 patients. A revised total in November 2017 suggested that he might have killed 106 patients. Finally, in January 2018, he was charged with murdering 97 patients.

In a trial held between 2018 and 2019, Högel confessed to 43 of the murders, while denying involvement in others. On June 6, 2019, he was found guilty of 85 counts of murder. The court characterized his motives as low and base. Further legal actions against his former supervisors ensued, questioning their neglect in addressing suspicions about Högel's conduct during his employment.