Former nurse Niels Hoegel has been convicted of 85 counts of murder and sentenced to life in prison. Picture: Hauke-Christian Dittrich/dpa via AP

Oldenburg, Germany - Serial killer Niels Hoegel has been sentenced to life in prison after a German court found him guilty on Thursday of the murder of 85 patients while working as a hospital nurse.

Hoegel, 42, had admitted to killing patients with lethal injections and was already serving a life sentence after being found guilty in 2015 of murdering two people.

"It seemed to me that I was death's bookkeeper," presiding judge Sebastian Buehrmann said as he read out judgement.

Hoegel initially selected his victims carefully in hospitals in the cities of Oldenburg and Delmenhorst in north-western Germany, injecting them with medication that led to heart failure or other complications between 2000 and 2005.

But his killings later became random.

Prosecutors claimed he did this so he could try to resuscitate them, motivated by boredom or a desire to impress his colleagues with his medical skills.

Noting the seriousness of the case, the court virtually ruled out release from prison after 15 years, as is usual practice in Germany.

The charge sheet originally listed 100 murders, of which Hoegel admitted 43. He was acquitted in 15 cases.

Referring to these cases, Buehrmann said of the relatives: "We leave you with doubts that are surely painful for you, but we have at this point to disappoint you."

Frank Brinkers lost his father, but is one of those who remains in uncertainty. "This is very, very bitter. I've been through hell, and it's difficult to bear."

Brinkers had hoped for final clarity. "Apparently it was not to be," he said after the case concluded.

The verdict marked the climax of a seven-month trial that drew global attention, in what is thought to be the worst serial murder case in post-war Germany.

According to a psychological report submitted to the court in April, Hoegel showed signs of a disturbed personality, including a lack of shame, guilt, regret and empathy.

His patients' condition would suddenly deteriorate. Alarms would go off. Hoegel was the first at the bedside to initiate life-saving treatment.

And he was good at it. It seemed that he yearned for the acknowledgement of his colleagues.

Buehrmann highlighted a case directly after Hoegel's daughter was born. Soon afterwards, the nurse struck again, by manipulating his patient's medication.

"You wanted to hang on to that feeling of happiness, by sending another person to their death," the judge said.

The killing spree ended in 2005 when another nurse caught him in the act of injecting medication that had not been prescribed into a patient.

In his final words to the court on Wednesday, Hoegel asked for forgiveness from the relatives of his victims, saying that the trial had brought home to him how much suffering his "terrible crimes" had caused.

His defence team asked for 55 murders to be considered and 14 attempted murders, calling for acquittal in 31 cases, while prosecutors saw murder proved in 97 cases, with the other three charges failing on grounds of lack of evidence.

dpa