This is the first time the Strasbourg court has condemned Italy for human rights violations in a domestic violence case.
"Italian authorities failed to protect a mother and son because they did not take prompt action on a complaint concerning conjugal violence," the court said in a statement published on Thursday.
The woman, E.T, is a 42-year-old living in Remanzacco, northern Italy. She was stabbed in the chest several times by her husband in November 2013.
Her husband, named as A.T. by the court, also stabbed the couple's 19-year-old son who had tried to separate his parents, and the teen later died of his injuries.
The court said that Italy had discriminated against the woman in failing to respond adequately to her earlier reports of domestic violence. Authorities "underestimated the violence in question and thus essentially endorsed it," they said.
The Strasbourg ruling condemned Italy for violating articles two (right to life), three (prohibition of inhumane or degrading treatment) and 14 (prohibition of discrimination) of the European Convention on Human Rights.
She had reported her husband's abusive behaviour to police multiple times between June and September 2012. Judges ordered Italy to pay her €40,000 - €30,000 in damages and a further €10,000 for legal costs.
The man's alleged abuses against his wife included beating her and her daughter, forcing her at knifepoint to have sex with his friends, and multiple threats of violence.
At one point, she was taken in by a women's refuge, but after three months had to leave due to "lack of available space and resources", the court said.
On the night of the stabbing, she had called the police after an argument with her husband and he had been taken to hospital in a state of intoxication.
After being discharged, police carried out an identity check on him as he was wandering along a street at around 2:25am, but let him go home after an on-the-spot fine, approximately two and a half hours before he carried out the attack on his wife and son.
The judges said that the failure of the authorities to react appropriately or in a timely manner to the initial reports of domestic violence "created a situation of impunity which contributed to repeated acts of violence, which in the end led to the attempted murder of the claimant and the death of her son".
"The national authorities should have taken account of the claimant's situation of great insecurity and moral, physical and material vulnerability, assessed that situation accordingly, and provided her with appropriate support, which they had failed to do," they concluded.
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A projection reading 'Stop Violence Against Women' on Rome's Campidoglio. Photo: AFP