Aggie Di Mauro believes Luay Sako, 39, should have received a life prison sentence without parole after he admitted to murdering her daughter Celeste Manno on November 16, 2020.
Ms Manno had been asleep in her bed in Melbourne's northeast when Sako smashed through her bedroom window and stabbed her 23 times.
Supreme Court Justice Jane Dixon on Thursday described the killing as "utterly cowardly" before sentencing Sako to 36 years behind bars.
He will be eligible for parole after 30 years and has already served more than three years in custody.
Celeste Manno was asleep in bed when she was attacked. (James Ross/AAP PHOTOS)
Ms Di Mauro said her family was devastated the justice system had failed her daughter again.
"The court decided to grant him mercy even though he showed Celeste none," she told reporters outside court.
"Quite clearly, his right to mercy was more important than her right to life."
During last month's plea hearing, the court was told Ms Manno and Sako were briefly co-workers but he began harassing and stalking her after she turned him down romantically.
Over a 12-month period, Sako sent her more than 140 messages from a number of Instagram accounts, with the communication becoming increasingly vulgar and degrading.
Ms Manno went to police and obtained an intervention order against Sako, but the contact only ceased after he was charged with breaching the order.
The day after he was arrested, Sako bought the knife he would use to kill Ms Manno three months later, prosecutor Patrick Bourke said.
Sako used Instagram and Google Earth to locate Ms Manno's home and drove past the property at least three times.
He also searched online for the home's floor plan so he could determine where her bedroom was.
On the night before her death, Ms Manno posted a photo of her boyfriend for the first time on Instagram.
A few hours later, Sako broke into her bedroom while she was sleeping and killed her.
Luay Sako admitted murdering Celeste Manno in her Melbourne home. (Diego Fedele/AAP PHOTOS)
Justice Dixon said the Instagram photo was the "final trigger" for an obsessed and jealous Sako, and the 39-year-old carried out the killing with "chilling efficiency".
"Celeste deserved life but you decided otherwise," Justice Dixon said.
In sentencing, the justice took into account Sako's diagnosed severe personality disorder, major depressive disorder and body dysmorphic disorder.
She said the conditions impaired his mental function but he was still able to understand the crimes he was committing.
A psychiatrist told the court Sako's prospects of rehabilitation were limited because he needed intensive treatment that was not available in the prison system.
But Justice Dixon said she was not persuaded that wouldn't change and his condition would remain untreatable.
She noted while the murder of Ms Manno was a grave example of a grave offence, it did not warrant a sentence of life in prison.
Ms Manno's family dropped their heads and started crying in the courtroom as the sentence was handed down.
An illuminated blue urn carrying Ms Manno's ashes was held by Ms Di Mauro throughout the sentencing hearing.
Aggie Di Mauro hopes prosecutors will appeal the sentence. (Con Chronis/AAP PHOTOS)
Outside court, Ms Di Mauro said she hoped the Director of Public Prosecutions and the Court of Appeal recognised the "true justice" in the case.
She said she would keep pushing for change, noting Victorian Attorney-General Jaclyn Symes had previously met with her over stalking reforms but ignored her recent messages.
"There were lots of promises made - none of them honoured," Ms Di Mauro said outside of court.
"All of them to look the part, say what needed to be said and then dishonour my daughter."
Ms Symes has been approached for comment.
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