Sako had worked with the 23-year-old at a Serco call centre in Melbourne's northeast for only a few months before he was fired.
Ms Manno was kind to him after his sacking so he became infatuated with her, creating an Instagram account so he could message her and profess his love.
Ms Manno's mother Aggie Di Mauro and other family members sat in court listening to her killer. (Con Chronis/AAP PHOTOS)
When she turned him down and then blocked his account to stop his incessant messaging, Sako merely created more accounts to harass her.
Over a 12-month period, he sent more than 140 messages to Ms Manno, with the texts becoming increasingly vulgar and degrading.
She went to police and obtained an interim intervention order against Sako but it didn't stop him from contacting her.
He told Ms Manno he would never hurt a soul and queried why she would see him as a threat.
But he killed her on November 16, 2020 - just hours after she posted a photo of her boyfriend for the first time on Instagram.
Sako drove to her Mernda home, scaled a fence to get into her backyard and then smashed her bedroom window with a hammer.
He broke into the room where Ms Manno was sleeping and stabbed her 23 times with a kitchen knife before going back to his car and driving to the police station.
The whole attack lasted under three minutes.
Sako pleaded guilty to Ms Manno's murder in the Victorian Supreme Court on Monday but he argued he did not stab her 23 times.
The 39-year-old, who is representing himself, claimed some of the wounds were caused glass from the smashed window.
Forensic pathologist Paul Bedford told the court the majority of Ms Manno's injuries would have been caused by a knife and there was no glass found in her wounds.
Sako is due to question Dr Bedford on Monday afternoon.
Ms Manno's mother Aggie Di Mauro and other family members sat in court listening to her killer, before they were due to deliver victim impact statements.
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