Patricia Dagorn, 57, is already serving a five-year prison term for theft, fraud and sequestration involving an octogenarian in the French Alps in 2012 after he agreed to let her live with him in exchange for sexual relations.
The 87-year-old man, Robert Mazereau, was later assaulted.
The year before, police had already suspected Dagorn after the body of Michel Kneffel, a man in his 60s with whom she had been living at a residential hotel in Nice, was discovered in July 2011.
No charges were filed in that case, but the investigation was reopened after her conviction the following year, when police found vials of Valium and personal documents belonging to about a dozen different men, including IDs, bank account details and health insurance cards.
That led investigators to another suspected murder case, that of Francesco Filippone, 85, whose body was found in his bathtub in an advanced state of decomposition in Mouans-Sartoux, outside Cannes, in February 2011.
Patricia Dagorn. Photo: AFP
Dagorn had earlier cashed a cheque from Filippone for 21,000 euros ($25,600), money which she said was a gift to help her open a jewellery shop.
"She denies the charges against her in their entirety, including the thefts," said one of her lawyers, Georges Rimondi.
When Dagorn was charged over the cases in 2015, Rimondi had described his client as a "fragile" individual who had been placed in foster care at a young age, and who says "she feels better with elderly people".
"She is eager to explain herself," said her other lawyer, Cedric Huissoud, adding that she had suffered during her years in prison.
'Quick and easy money'
Police now think Dagorn may have met at least 20 men after arriving on the French Riviera in 2011, mainly via a matchmaking agency.
In most of the cases she allegedly asked the men for money or to name her in their wills, or she stole documents from them, while a few were accused of rape.
Two of the men, Ange Pisciotta, 82, and Robert Vaux (see below), 91, were to testify at the trial in Nice.
Victim, Robert Vaux. Photo: AFP
Vaux, who had brought Dagorn to live with him in early 2012 in the coastal town of Frejus, saw his health deteriorate rapidly just as she was making arrangements with his solicitor.
Dagorn, who has a law degree, had already been handed a one-year suspended prison term in a case involving her ex-husband, who was found guilty of fraud.
In 2013, her youngest son told a local newspaper that he was not surprised by the accusations against his mother.
"She has always been obsessed with quick and easy money," the son, identified as Guilhem, told Nice-Matin.