Niels Högel, an ex-nurse in Germany, has admitted killing 100 patients by delivering them a fatal dose of medication.
Mr Högel, who is already serving a life term for the death of six patients in his care, made the admission at the start of his new trial.
He said he was trying to impress his colleagues by resuscitating the victims he had just attacked.
It's now believed the deaths occurred at two hospitals in northern Germany between 1999 and 2005.
When asked by the judge whether the charges were correct, Mr Högel said "more or less".
Investigators believe the 41 year old could have killed even more, but potential victims have been cremated.
Mr Högel was first charged with attempted murder in 2005 after being caught injecting unprescribed medication into a patient.
He was sentenced to seven years in prison in 2008 and then in 2014 he was charged with two murders and two further attempted murders.
Investigators widened their search into Mr Högel's past when he confessed to a psychiatrist that he had killed up to 30 people.
It's expected that the current trial will last until May next year.