Nicola Gobbo may have turned informer because she felt guilty about not preventing a double murder, a former Victorian detective believes.
Gavan Ryan headed the task force investigating the murders of police informer Terence Hodson and his wife Christine in 2004.
"My hunch is she knew something was going to happen to them," he revealed at an inquiry into police use of informers on Wednesday.
"She regretted not doing something and decided to 'roll over'."
Ms Gobbo, now known as Lawyer X, was registered as a police informer in 2005, a year after the Hodsons were murdered execution-style in their Kew home.
Former policeman Paul Dale and hitman Rodney Collins were charged with the murder, although charges were later dropped.
Mr Hodson had turned informer against Mr Dale over a burglary on a drug stash house.
Charges against Mr Dale in that case were dropped when the Hodsons were killed.
Mr Ryan said he once asked Ms Gobbo directly at a dinner with other police and handlers what her motivation for informing was, but she fobbed him off.
Commissioner Margaret McMurdo acknowledged Mr Ryan's theory was just that - a theory.
"But it's an interesting one," she said.
The commission also heard more about Ms Gobbo's evidence at secret Office of Police Integrity hearings in 2007.
Evidence before the commission revealed Mr Ryan had a conversation on the day of her second appearance with then-head of the Office of Police Integrity - and current Victoria Police chief - Graham Ashton, about Ms Gobbo being an informer and fears she'd be compromised.
Mr Ryan couldn't remember any conversations with Mr Ashton that day, but diary notes reveal he told Mr Ashton about Ms Gobbo's value to Victoria Police and her potential to provide them with information about the Hodson murders and the 2003 murder of Shane Chartress-Abbott.
He's also recorded as asking Mr Ashton to give consideration to possible damage to her relationship with Victoria Police if she was forced to answer questions about sexual relationships she had with police - but she was still asked.
The commission heard Mr Ashton believes that conversation was the first time he learned Ms Gobbo was an informer, but last week it was said he had been told by Mr Overland in April 2006.
Issues later arose when Ms Gobbo, who told her handlers she was a witness, was asked to identify anybody she told. She feared telling the truth would expose her.
After a tearful break during those 2007 hearings, Ms Gobbo said she had only told Mr Ryan and one other person, which Mr Ryan now admits was misleading given a number of people knew.
Hearings were suspended when OPI chairman, former Queensland judge Tony Fitzgerald, accused her of lying and suggested she get legal representation.
The commission was told Mr Overland approved payment by Victoria Police for Ms Gobbo's representation, but the hearings never resumed.