Police missed "clear opportunities" to arrest an assault suspect days before he went on to brutally murder his victim, a report says.
Paul Cooper suffered a "brutal and prolonged" attack
Paul Cooper called Greater Manchester Police after he was attacked by Neil Read, 24, in February and March 2005.
But a six-day delay in putting Read's name onto a database meant he slipped through the net, the Independent Police Complaints Commission (IPCC) found.
GMP said the two officers involved had been given written warnings.
Mr Cooper, 42, from Heywood, Greater Manchester, gave officers the name of Neil Read on 9 March.
The IPCC said the initial failing was that information about Read was not entered onto the Greater Manchester Police Intelligence Computer System until 15 March.
Read was then arrested on another matter on 11 March but was released on bail because there was no information to say he was wanted over the assaults.
Officers later found out that Read was due to appear before Rochdale Magistrates on 16 March.
Although two officers attended the court in the morning they left when they found out he was not due to appear until the afternoon.
Read murdered Mr Cooper on 18 March and was last year ordered to be detained indefinitely at Ashworth high security hospital.
The IPCC investigation identified a "lack of professionalism" in the officers' handling of the case.
Mr Cooper had been the subject of an "horrendous campaign of violence" by Read, who wrongly thought he was a paedophile.
Mr Cooper was found dead at his flat in Heywood
Police said there was no suggestion Mr Cooper was involved in sexual offences.
Naseem Malik, IPCC Commissioner for the North West, said the two officers did investigate the earlier assaults professionally, even arranging for a panic alarm to be fitted into Mr Cooper's home.
"However it is also clear that there were unacceptable failings by those officers once the suspect was identified which meant clear opportunities to facilitate Read's arrest were missed.
"Read's mental illness was the catalyst behind the murder and due to this we can never know what his response would have been to earlier police intervention.
"However Greater Manchester Police did owe it to Mr Cooper to act quickly and decisively to confront a clear threat and they failed to do this."
A spokesman for Greater Manchester Police said the force welcomed the "comprehensive and extensive" review into the death of Mr Cooper, which also highlighted the good work carried out by officers.
"The IPCC highlights that the officers concerned acted professionally and decisively when the assaults were reported to GMP. This included the installation of a panic alarm at the house.
"Sadly their hard work did not prevent Neil Read from murdering Paul, which is something they and GMP deeply regret.
The spokesman added that the force had addressed the two shortcomings identified by the IPCC, which GMP says were at "the lower end of the disciplinary scale", and had given the officers management advice and warning letters.