The family of Colin Marr claim police attempted to "cover up" their failings over the investigation into his death.
The 23-year-old's mother, Margaret Graham, insists senior officers made "conscious decisions" not to launch an internal investigation into their shortcomings.
His mother also accused Fife Constabulary of trying to "expunge" the case by destroying DNA evidence.
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Margaret Graham, of Inverkeithing in Fife, told the Sunday Herald: "It has been excruciating to read the lies by the people who we are meant to trust. Colin did not deserve to be treated with such disdain."
The Police Investigations & Review Commissioner's findings, expected to be released publicly tomorrow, reveal a catalogue of errors by officers.
Margaret Graham said: "We raised failings in 2008 and no one from Fife police would acknowledge them. This is conscious decision-making - not a mistake. If they had taken action many forensic elements may not have been lost. Fife police then took it upon themselves to destroy DNA. This is consistent with a theme of Fife police wanting to expunge Colin's case and make it go away. By failing to contact us prior to the funeral, the police showed a complete disinterest at an early stage. Fife police never shared with us that there was a fight in the house that night. If we had known, we would have stopped the cremation."
Marr's stepfather Stuart Graham has been a key part of the family's fight for justice, even standing in the Cowdenbeath by-election last year as an independent candidate. He said: "The police have consistently denied their failings."
The death is being treated as unresolved and the case remains open. Marr's family recently submitted new reports to the procurator fiscal but Stuart Graham has lost confidence in the police. He said: "How can we trust Police Scotland on any aspect of criminality in this case."
A Crown Office spokesman confirmed they received correspondence from the family of Colin Marr. He added: "It is under consideration by the procurator fiscal."
A Police Scotland spokeswoman said: "We welcome the report from PIRC. Police Scotland has engaged with Mr and Mrs Graham.
"Former Fife Constabulary acknowledged its failings following a complaint investigation and issued a formal apology to Mr and Mrs Graham. A subsequent complaint investigation concluded in March 2013. On this occasion, Police Scotland referred Mr and Mrs Graham's complaint to PIRC. We have now received the Case Handling Review from PIRC. The recommendations contained within will now be considered as part of Police Scotland's commitment to continually improving our service."