The case has brought back memories of Niels Högel, a German nurse sentenced to life in prison earlier this year for murdering 85 patients.
The defendant on trial in Munich, who has been named by prosecutors only as Grzegorz Stanislaw W., 38, is also accused of three attempted murders.
The court heard that the man had access to insulin because he has diabetes – unlike his patients. He has also been charged on multiple counts of stealing from his victims.
At the start of the hearing, the judge said he faced a life sentence with a minimum of 15 years in prison.
The man visited the homes of at least 69 patients between April 2017 and February 2018, leading to media speculation that more victims could be found.
His victims were in different parts of Germany from Bavaria in the south to Hanover in the north.
Prosecutors said the man hated his job and committed one of the murders in order to avoid being sacked.
According to prosecutors, he told his psychiatrist in prison that he was “really sorry” and felt remorse.
The court also heard that he had been convicted of fraud in Poland and was in prison between 2008 and 2014.
The defendant chose not to speak on the first day of the trial, with his lawyer Birgit Schwerdt telling the court that he did not want to “give any information on his personal circumstances or the case”.
The trial is scheduled to last until May 2020.
Högel, believed to be Germany's most prolific serial killer, murdered patients with lethal injections between 2000 and 2005, before he was eventually caught in the act.