Thanh Ngoc Truong broke up with her boyfriend, 30 years her senior, because he was too controlling and overbearing.
A month later, he murdered her with a double-barrelled shotgun in her home, leaving the 48-year-old's daughter to discover the gruesome scene.
Juan Gonzalez, 77, shot Ms Truong once in the head, killing her instantly, before firing another round into her neck at her St Albans unit in May last year.
Her body was unrecognisable, paramedics later said.
Gonzalez went home and put his gun back in its safe before handing himself in to police. He said he shot his girlfriend in the head and that while he felt guilty she had "wanted to destroy him".
He said Ms Truong had been using a psychological ploy that made him feel crazy.
Gonzalez on Thursday faced the Victorian Supreme Court, having pleaded guilty to Ms Truong's murder.
"The offending is so serious - it is almost inexplicable in an otherwise long and blameless life," his barrister Carly Marcs said.
Prosecutor Mark Rochford QC said the brutal and premeditated killing followed a controlling and abusive relationship.
"This was planned - he had to open safe, get the gun, load it, transport it, keep it away from members of public ... then enter the house and pull the trigger from close range," Mr Rochford told the court.
"She should have been able to feel safe in her own home."
Mr Rochford also said there was no evidence Ms Truong had made any demands of Gonzalez following the end of their relationship, other than what the 77-year-old told police.
The prosecutor described this as a self-serving statement "akin to victim-blaming or 'she made me do it'".
Gonzalez will be sentenced at a later date.
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