Daniela Poggiali was arrested in October 2014 in Lugo, northern Italy, and sentenced to life behind bars in March last year. A court acquitted her on Friday.
Publication in the media of pictures of her smiling next to recently deceased patients had caused uproar in Italy, amid reports that she had given huge doses of drugs to sick patients she found "annoying".
At the emergency room where she worked she was always engaging and tireless with patients' families, but seems cold and unpleasant with colleagues.
And in the first three months of 2014, 38 of the 83 deaths registered in her department occurred when she was on duty, against an average of no more than 10 for her fellow nurses.
At the start of April a series of troubling coincidences alerted the authorities. And when a 78-year-old woman died shortly after being taken care of by the nurse, an investigation was launched.
The autopsy showed that she had been given a massive dose of potassium chloride. Prosecutors said it could not have been a mistake, and Poggiali was the only person who could have given the injection.
Potassium chloride is detectable in the body only for a few days after death, therefore the other suspicious cases raised by prosecutors -- around 10 over a period of a few months -- could not be added to the official file.
But an expert called by Poggiali's lawyers during the trial over the death she was convicted of found that, if the injection had occurred as claimed, the patient would have died in a matter of minutes, and not over an hour as was the case.
"The facts do not stand up," the Bologne appeal court ruled on Friday.
The former nurse was released on Friday evening, after nearly three years behind bars.
"They described me as someone who I am not, and now I am going to be able to get my life back again," she told reporters.