The 36-year-old suspect, who for years worked as a temporary nursing assistant in private homes in Germany, was also charged with three counts of attempted murder and three counts of aggravated assault.
The man, who had previously served jail time in Poland for property offences, had worked in neighbouring Germany since May 2015 as a temp nursing assistant via Polish and Slovak agencies.
A diabetic, he used insulin to kill his victims, said Munich police in a statement.
The suspect had admitted to administering insulin in the 12 known cases since April 2017, but claimed he did not mean to kill.
He was arrested in Munich early this year, and police said they had since investigated 68 locations where he worked.
He stole mostly cash and jewellery but also household goods – and even food- in the crimes spanning the states of Bavaria, Schleswig-Holstein and Lower Saxony, said the statement.
If there was nothing valuable to take, he would quickly leave, claiming there had been a death or serious illness in his family, said police.
News of the case comes weeks after the start of the shocking new trial of former German hospital nurse Niels Högel, who has admitted to killing 100 patients in his care, the biggest serial killing case in the country's post-war history.
Högel, 41, who has already spent nearly a decade in prison for other patient deaths, is accused of intentionally administering medical overdoses to his random victims so he could bring them back to life at the last moment.
Prosecutors say he was motivated by vanity, to show off his skills at saving human lives, and by simple “boredom”.