Mark James Bombara, 63, shot dead Jennifer Petelczyc, 59, and her daughter, 18-year-old Gretl, at their Perth home on May 24, 2024, before killing himself.
An inquest has heard Bombara had been searching for his estranged wife Rowena, after they separated a month earlier, but she wasn't at the home.
Bombara's daughter Ariel says the "absolute terror and fear" the Petelczycs likely experienced should "devastate" the community.
"I hope our collective rage at what my father did to them is the wake-up call we need to spark real change, as these unprecedented rates of family and domestic violence cannot continue," she told the WA Coroners Court on Friday.
My father was not "the gentle, sad old man" and a victim of poor health, as he "conned many into believing", she said.
"He was a devious, ruthless manipulator who felt so entitled to my mum's energy and enduring unpaid labour.
"The punishment for her leaving his abuse could be nothing other than her murder."
When Bombara realised Rowena wasn't at her best friend Jennifer's home, he murdered two women as "the next best punishment".
"I would never have predicted that he wouldn't do his due diligence in confirming mum's location before attempting her murder," she said.
Ariel apologised to Jennifer's surviving daughter, Liesl.
"I am so deeply, truly sorry for what my father has done to you," she said.
Ariel was critical of police for allowing Bombara to retain 13 firearms despite multiple warnings about his mental state and potential for violence.
"For the rest of my life I will feel like I pulled the pin on the grenade that landed on someone else's family," she said.
She also apologised to her mother, Rowena, for not "being able to save her from this grief" after she heard Bombara murder the Petelczycs via a telephone call.
"Her scream after hearing those shots, and her wails ... will haunt me for the rest of my life, she said.
The inquest has heard Bombara was married to Rowena for 35 years and he became fixated on her after the split.
His murderous rampage at the Petelczyc's home was recorded in eight triple zero calls and CCTV.
Acting State Coroner Sarah Linton will hand down her findings on a date to be fixed.
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