Killer dad Rowan Baxter planned to murder his ex-partner and son from a previous relationship years before killing his wife Hannah Clarke and their three kids.

Nikki Brooks, Hannah's best friend, shared chilling details of Baxter's history of violence - and revealed how the abusive husband boasted about his aggressive behaviour to his own wife.

The 42-year-old told her of his evil plot to kill his ex-girlfriend and their son Isaiah back when the relationship had just ended and even admitted he bought a rope to carry out the murders.

'He'd driven to do it. He'd prepared what he needed to kill Isaiah and [Isaiah's mother] and then didn't do it,' Ms Brooks told The Australian.

'Isaiah would have been maybe seven or eight. It was the first time she was going to leave him as well.'

Scroll down for video

Rowan Baxter had told Hannah of his evil plot to kill his ex-girlfriend and their son Isaiah (centre) back when the relationship had just ended and even admitted he bought a rope to carry out the murders, a friend revealed

The 42-year-old (pictured with his family) is believed to have been abusive to his wife Hannah Clarke for a decade before she took out a domestic violence order against him

Baxter had also told Hannah about a separate incident during which he nearly beat someone to death after he 'lost his mind' in a road rage attack when Isaiah was still a baby.

He was later convicted of grievous bodily harm over the incident.

Isaiah Baxter, now 22, shared his support for the Clarke family in the wake of the horrific murder-suicide, while speaking out against his killer father.

The young man revealed he too had been a victim of abuse at the hands of Baxter growing up, saying 'his appalling parenting caused irreversible damage to my mental health'.

'If I could take the place of Laianah, Aaliyah, and Trey I would in a heartbeat,' he said in a heartbreaking letter to the family.

Friends have since come forward with stories of Baxter's abusive and controlling behaviour over the years and up until his horrific attack.

Rowan Baxter had an obsession with porn and was abusive to his mum growing up, his cousin revealed

Alana Hampson revealed her cousin Rowan showed 'little hints' of cruelty throughout his life and had a particular aggression and contempt towards women as a child

'I knew he had it in him. I just thought, hopefully, any person would be able to pull themselves back,' Ms Brooks added.

Baxter's cousin Alana Hampson, who lived with him in New Zealand after she ran away from home as a teen, said Rowan had 'dark secrets' that turned him into a cruel monster capable of killing his three children.

'He was a monster, even when he was a kid, I think he was a monster... He was very controlling and manipulative,' Ms Hampson told Newshub.

'I didn't like to be around him because I felt uncomfortable. The way Rowan used to speak to his mother was... disrespectful.'

Ms Hampson said Baxter had an obsession with sex and treated women with contempt from a very young age.

'I think that Rowan should have been going through counselling as a kid,' she said.

'The kids got into porn, always looking at porn [and] his mum accepted it, he treated his mum like c**p. She was a nice lady.'

Baxter burned his estranged wife Hannah Clarke and their three young children alive in their family car in a vicious act of domestic violence before he killed himself at Camp Hill, Brisbane, last week

Nikki Brooks, Hannah's best friend, revealed how Baxter would boast about his aggressive behaviour to his own wife

Baxter's mum died of a brain aneurysm when he was in his 20s which Ms Hampson believes sent him on a downward spiral.

She revealed Baxter himself was a victim of abuse, started having sex at a young age and had an 'ugly' life growing up, but was never able to change for the better.

The 42-year-old is believed to have been abusive to his wife Hannah Clarke for a decade before she took out a domestic violence order against him.

He lost custody of his children in February after he breached the order, and just a couple days later, killed them and his wife by setting them on fire.

Ms Hampson had previously told Daily Mail Australia Baxter had shown a particular aggression and contempt towards women as a child.

'There was a lot of violence and rough play... Rowan was disrespectful to women and talked dirty to females... He was f***ed in the head,' she said.

Her mother, Dorothy Ann Baxter, is the sister of Rowan's father.

Dorothy Ann described her nephew as 'a good boy' who must have 'lost the plot'.

'He's lost the plot. I don't know. He couldn't handle it then one day he just lost it ... But I know he wouldn't do it out of his own mind so he had to be … just lost the plot ... I'm not saying Rowan was good for what he did but people do lose the plot,' she said.

It comes after another cousin on Rowan's mother's side, Sandra Taylor, told Daily Mail Australia he had a 'dangerous sense of possession and entitlement over his wife and children'.

Rowan Baxter's cousin Sandra Taylor (pictured) told Daily Mail Australia she feared he would murder his estranged wife Hannah Clarke

'Rowan believed that women are two things - to be a house cleaner, and to be a prostitute,' the mother-of-one said.

'This was a man with a level of hatred and disrespect for their mother so great that he would make this choice - the most horrifying and despicable of choices.'

She said that while she was 'shaken to the core' upon learning what her cousin had done, she was not surprised.

Baxter burned his estranged wife Hannah Clarke and their three young children alive in their family car in a vicious act of domestic violence before he killed himself at Camp Hill, Brisbane, last week.

Laianah, aged four, Aaliyah, six, and Trey, three, died on the scene. Hannah died in hospital on Wednesday after suffering horrific burns to 97 per cent of her body.

Three days before the quadruple murder-suicide, Baxter told a friend he wouldn't do 'anything stupid', the Sydney Morning Herald reported.

The pair spoke for two hours as the friend feared Baxter was suicidal after his estranged wife Hannah Clarke took out a Domestic Violence Order against him.

Baxter, 31, jumped out of the white SUV (pictured) engulfed in flames yelling 'he's poured petrol on me' and was hosed down by a heroic bystander

During the conversation Baxter revealed he was devastated that he was not allowed to see his children.

Talking about Ms Clarke, he said 'she can do whatever she wants to me but don't take my kids away from me, that's all I've got left in life,' the friend told the newspaper.

The friend also revealed that when they parted ways after the meet-up he warned Baxter not to 'do anything stupid'.

'The last thing I said to him as he was walking away was 'mate, don't do anything stupid' and he sort of turned as he was walking away and he just said 'no mate, I'm not going to do anything stupid.' I was sort of thinking that he might self-harm,' he said.

The friend said that Baxter was suffering extreme depression that drove him to become a 'monster' three days later.

'If someone told me Rowan was capable of that, I would say, no way, not to his kids. He loved his kids more than anything else in the world,' the friend said.