Catherine Thomson, 26, was stabbed in the jugular vein at her family home in Moodiesburn, North Lanarkshire.
John Campbell attacked Ms Thomson - his brother's girlfriend - while on leave from Castle Huntly prison in 2005.
A fatal accident inquiry ruled that a risk assessment should have been carried out before he was given leave.
Sheriff Thomas Millar ruled Ms Thomson's death could have been avoided.
Campbell was arrested and later plunged to his death from an upper gallery at Glasgow's Barlinnie prison, in an apparent suicide.
Lawyer Cameron Fyfe said Ms Thomson's family were taking legal action against the Scottish Prison Service (SPS).
Mr Fyfe, who is acting for the family, said: "We take the view that the determination from the sheriff gives us the basis to proceed on the basis that the prison service was negligent in allowing Campbell out without a proper risk assessment.
"The family really don't care about the compensation money, they haven't even asked me how much they could get.
"What they want, in their words, is for some justice to come out of this and for the prison service to pay for the mistake that they made."
Campbell, 34, was serving an eight-year sentence for two charges of assault to severe injury and permanent disfigurement when he was allowed out of prison on short unsupervised leave.
He had initially been placed on high supervision level when he was sentenced in August 2002.
In 2004 his supervision level was wrongly reduced to low, which meant he was eligible for unsupervised home leave when he was transferred to Castle Huntley prison.
Sheriff Millar found that officers there had assumed Campbell was of low risk to the community because he was on a low supervision level.He added they failed to carry out a fresh risk assessment on Campbell because they assumed one had been carried out elsewhere and were simply "rubber-stamping" it.