Judge Michael Höhne determined in his ruling that the victim, Alex H., had been tied down and wearing swimming goggles covered in duct tape at the time of the crime, which took place in late 2008.
At this point Gabi P. twice pressed the electric saw against his throat and then pressed it against his chest, leading to his death.
The fact that the victim was both unsuspecting and defenceless at the time would normally be grounds for a murder conviction, the Süddeutsche Zeitung reports.
But Höhne ruled that it could not be proved that Gabi P. planned to pick up the saw, which was lying in the bedroom after the couple had done some renovation work, or whether she had spontaneously reached for it during the sex game.
The court nonetheless handed down a 12-and-a-half-year sentence on the lesser manslaughter charge.
Prosecutors had demanded a life sentence, saying that the attack was premeditated. They cited Gabi P.'s diary entries, which recorded her tumultuous relationship with Alex H. and the demeaning sexual practices he thought up and which she let happen.
Investigators spoke to witnesses who recounted how Gabi P. threw her boyfriend out of the house after a huge argument at some point in December 2008. But shortly after, he was able to persuade her to let him move back in.
The prosecution alleged that it was at this point the young woman decided to murder him.
But her defence insisted that the act was not premeditated and called for a ten-year sentence for manslaughter. They argued that it was pure speculation that the victim was bound and blindfolded at the time of the crime.
Gabi P. told the court that she could not remember the events leading up to the killing, only that she had pushed the saw towards her boyfriend.
Alex H.'s body was only found in 2016, after Gabi P. buried it in the garden of their house at some point after the crime. His parents presumed that he had run away to Romania.