A police officer has told the inquest into Luke Batty's death that he always believed the boy's father was capable of murder.
Greg Anderson was "100 per cent bad" and always likely to kill his son, according to a police officer who says his efforts to have him locked up were undone by the courts.
Senior Constable Paul Topham told the inquest into Luke Batty's death on Thursday that Anderson's behaviour had become increasingly erratic, and he knew the 54-year-old was capable of killing.
"If he didn't do it on the 12th of February, he would've done it further down the track," he told the Victorian Coroners Court.
"The writing was on the wall."
Anderson clubbed his 11-year-old son, Luke, with a cricket bat and attacked him with a knife after cricket training at Tyabb oval, southeast of Melbourne, on February 12.
Sen Const Topham arrested and interviewed Anderson in January 2013 after he assaulted Luke's mother, Rosie Batty, and was told repeatedly by Anderson, "You have nothing, I wish to leave."
The interview ended when Anderson sat in the corner of the room, facing the wall.
In a court appearance the following day, Sen Const Topham opposed bail and was stunned when the magistrate released Anderson, despite the defendant's courtroom rant and his lack of a fixed address.
"He is nuts," Sen Const Topham said of Anderson in an email to a colleague.
"He is contesting all matters and will only answer to Jesus. He even told the magistrate that and still got bail."
Sen Const Topham said police had worked hard to arrest Anderson and keep him in custody, but were continually thwarted by the courts and mental health system.
"What do we do with someone like this?" he told the inquest on Thursday.
"Police weren't mucking around on this bloke. We were trying to get him.
"I had serious concerns for the victim and I wanted Anderson off the street."
Anderson also knew how to exploit legal loopholes, according to Sen Const Topham.
He said Anderson would deliberately miss court dates, knowing an arrest warrant would be issued and supersede his bail conditions, meaning he no longer needed to report weekly to police.
"He was 100 per cent bad, not mad. He knew what he was doing," Sen Const Topham said.
Ms Batty had to be comforted by her lawyers while watching Sen Const Topham's interview with Anderson 13 months before he killed Luke, which was screened in court.
Ms Batty later stormed out of court when Sergeant John Schroen gave evidence that she had been in a pleasant mood when he arrested Anderson at the Tyabb oval on one occasion before Luke's death.
Ms Batty previously told the inquest efforts to arrest Anderson when he turned up at the oval had been traumatic.
The inquest continues.