Detective Sergeant Derek Harris said the long "suicide/goodbye" message he reviewed during his investigation into their fiery 2020 deaths indicated Baxter was a narcissist who took no responsibility for his actions.
"The note was written on two different dates. It was two different personalities," he told the inquest on Friday.
"When you read it, it is like someone who is not with it.
"It was someone ... who had given up, who was looking for an out, looking to blame everyone but himself."
Details of the note to Ms Clarke were first revealed at the inquest on Thursday.
In it, Baxter wrote: "I'm finishing your game. I don't want to play anymore. I have told the kids that you loved them. They will miss you, I'm sure."
Det Sgt Harris also reviewed almost 300 statements as part of his investigation into the lead-up to Ms Clarke and her children's deaths in February 2020, which again painted an unflattering portrait of Baxter.
The deaths of Ms Clarke, her children - Aaliyah, six, Laianah, four, and Trey, three - and Baxter are the subject of the inquest that started in Brisbane on Monday.
Det Sgt Harris said Ms Clarke lived in an "oppressive regime" under Baxter. After they separated in late 2019, he played the victim, creating an "echo chamber" where he repeated his lies enough times that he believed them.
"Everything she did was wrong. She couldn't go anywhere, wear what she wanted, speak to friends she wanted - it was ultimate control over every aspect of her life," the detective said of the marriage.
Months before the deaths, Det Sgt Harris received a "child harm" referral after Baxter abducted one of his daughters on Boxing Day 2019.
Baxter grabbed Laianah and put her in his car so quickly she bumped her head, leaving a "very distressed" Ms Clarke and their other children behind "wailing", a witness told the inquest on Friday.
The witness, who didn't know the family, was so upset by what she saw she felt compelled to report it to police.
"A loving father who cared for his child wouldn't behave that way," witness Julie Ann Humphries said.
Baxter took his daughter to NSW for more than two days before Laianah was returned to Ms Clarke following police intervention.
Det Sgt Harris - from Morningside Child Protection Investigation Unit - said what Baxter had done was "disgusting" but "wasn't a crime".
He reviewed the matter after the daughter had been returned and a domestic violence order was in place.
He considered it a custody dispute and referred it to the Department of Child Safety.
"There's no family law court order, there was no child protection orders in place to stop Rowan legally having access to that child (on Boxing Day)," Det Sgt Harris said.
Just weeks later on February 19, 2020, Ms Clarke was leaving her parents' home in Camp Hill to take her children to school when Baxter got into the car, poured fuel inside and set it alight.
Baxter, 42, then stabbed himself with a knife, dying nearby.
Ms Clarke, 31, died later the same day in hospital.
The inquest continues.
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