Hannah Clarke's parents believe nothing could have stopped Rowan Baxter killing their daughter and grandchildren, describing him as a "true monster".
However, Sue and Lloyd Clarke hope an "unthinkably confronting" inquest into the deaths will help others avoid the same fate.
The inquest was told there were systematic failings in police communication and training before Ms Clarke's estranged husband torched her and their three children in a car in suburban Brisbane in February 2020.
Deputy state coroner Jane Bentley has been asked to make recommendations to address the issues when she delivers findings in June.
However, Lloyd Clarke says nothing would have stopped Baxter.
"He was just one of those people ... so callous and used everyone as a pawn in his monstrous ways," he said outside the court.
Sue Clarke added: "Every now and then I think a true monster is born and you can't stop them".
Ms Clarke's parents were stoic when Ms Bentley saluted their bravery after enduring a harrowing nine-day inquest, forced to "relive the worst day of our lives in excruciating detail".
They struggled to control their emotions when delivering heartbreaking victim impact statements on Thursday, saying the inquest could be summed up in one question: why?
"(My grandchildren) ... would say 'why would you hurt us daddy? Why didn't you love us just like a father is supposed to?'," Lloyd Clarke said.
"Hannah would say 'why couldn't you be a better man, a better father, a better husband - why did you have to be such a coward and bully to us for so many years?'."
He added: "Why does it take the murder of four beautiful souls and dozens of others every year before governments respond?"
Sue Clarke told the inquest the children's room at her home remains untouched.
"Our house, once filled with the noise, laughter and chaos of young children is now very quiet.
"The hole in our family is gaping and will never be filled."
Counsel assisting the coroner Jacoba Brasch QC told the inquest she could answer the question why Baxter killed his family.
"Why? Because Baxter was evil," she said.
Dr Brasch believed it was only a matter of time before Baxter killed Ms Clarke once they separated in late 2019, such was his "single-minded, murderous intent".
"Hannah really was the walking dead the moment she asserted her own independence and left Baxter," she said.
Baxter committed "calculated and premeditated murder", Dr Brasch said, citing CCTV footage of him casually buying zip ties, cleaning fluid and a fuel can days before the killings.
The footage was among evidence Ms Clarke's parents had not previously seen.
Baxter bought three Kinder Surprise chocolates - presumably for his children - when he purchased fuel in the lead-up to the murders.
He may have initially intended to kidnap Ms Clarke, burn her, then give his children chocolates and it "would all be happy families," Dr Brasch said.
But he turned to "plan B" once the 31-year-old defied him and sought help from a bystander when he ambushed her, she told the inquest.
Ms Clarke was leaving her parents' home to take the children - Aaliyah, six, Laianah, four, and Trey, three - to school when Baxter jumped into her vehicle, splashed fuel and set it alight on February 19, 2020.
The 42-year-old then fatally stabbed himself.
Dr Brasch recommended more specialised training for officers, trialling a "multi-disciplinary, standalone" police station so victims could access specialist services, and addressing a lack of men's behavioural and anger management services.
Queensland Police Union barrister Ruth O'Gorman recommended more permanent positions at the Vulnerable Persons Unit, streamlining DV applications and information-sharing improvements between state and Australian jurisdictions.
Barrister Kylie Hillard, representing the Clarkes, said the family wanted changes to how frontline workers handle domestic violence suspects, including mandatory counselling or behavioural change courses as part of protection orders. They also want more state government-funded accommodation for families affected by domestic violence.
Sue Clarke said the inquest had given them "hope and a future".
"(Hannah) was a bright light in our lives and though she has been stolen from us, we will work every day to ensure that light never goes out."
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