Niels Högel injected drugs into his victims in order to give his patients heart attacks and receive praise from his colleagues for giving resuscitation.
The murders took place between 2000 and 2005 in the northern German towns of Delemhorst and Oldenburg.
The former nurse was in total accused of 100 murders, targeting patients aged between 34 and 96 years old, and acquitted of 15 cases on Thursday because of a lack of evidence.
Police say the final death toll could be as high as 200, but the Oldeburg district court heard on Thursday that police were unsure as to the real number because many of those killed were cremated before autopsies could verify whether or not Högel was responsible for their deaths.
He is already serving a life sentence for two murders and two attempted murders in 2015.
Judge Sebastian Bührmann said that the 42-year-old nurse committing crimes “beyond comprehension”.
"I felt like an accountant of death," Bührmann said on Thursday. "The fact is, sometimes the worst imagination is not enough to describe the truth."
The judge added that while there are no consecutive sentences in Germany, the “particular seriousness” of the crimes committed means Högel will all but certainly remain behind bars after the normal 15-year-term finishes.
On Wednesday Högel had apologised to the relatives of his victims and said he had come to realise the amount of suffering his “terrible deeds” had caused.
A lawyer for the victims’ relatives said his apology lacked credibility.
“He only acted out his remorse to gather plus points […] He should have stayed silent,” Gabby Lübben told Bild newspaper.
Experts believe that Hogel showed signs of having a personality disorder but wasn’t deemed mentally unfit.
According to psychiatric expert Henning Saß, Högel lacked shame, guilt, repentance and empathy.