Police officers who did not arrest a man could not have predicted that he would go on to kill hours later, the Ombudsman has concluded.
The finding came following an investigation by Police Ombudsman Michael Maguire.
Ahmed Noor stabbed Mohsin Bhatti, 29, in the Botanic Avenue area of south Belfast in the early hours of January 29, 2015.
Noor, 35, described in court as a paranoid schizophrenic, later pleaded guilty to manslaughter on the grounds of diminished responsibility.
He was given an indeterminate custodial sentence, and told that he must serve at least six years' detention before being considered eligible for release.
The case was referred to Mr Maguire's office, which has completed an independent investigation into the events leading up to the killing.
The Police Service of Northern Ireland (PSNI) were called to the scene around 10pm on January 28, 2015 after Noor had tried repeatedly to open the front door of a flat occupied by two women.
Inquiries by Police Ombudsman investigators found that four officers had arrived at the women's apartment within six minutes of receiving the call, during which time the man trying to gain entry to the flat had been described as "very disorientated".
One of the women said Noor had initially asked if he could stand in the porch. When this was refused, she said he stood in a nearby car park staring at the apartment, before returning and knocking the door again.
She and her flatmate refused to open the door, after which he went back to the car park, before returning and trying to open the door.
She described the man as "unstable", and said this happened four times before they called police.
When interviewed by Police Ombudsman investigators, the officers who responded to the call said Noor had been nervous, confused and "rambling incoherently", but was also compliant and unaggressive.
They said he refused to answer their questions, so they decided to search him under the Police and Criminal Evidence Act. Nothing was found.
The officers said that as he had committed no offences, and was not deemed to be a threat to himself or others, there were no grounds for arrest and they had no option but to allow him to leave the area.
Having reviewed the evidence, Police Ombudsman Dr Maguire concluded that the officers had acted appropriately.
He said they had "reasonable suspicion" to carry out a search, which they had done in accordance with PACE and police procedures.
"They described Noor as being agitated, confused and disorientated, but none of them considered him to represent a threat to himself or anyone else. Their decision not to arrest him was the right one in the circumstances. There were insufficient grounds for an arrest," said Dr Maguire.
"They could not have predicted that he would go on to kill just a few hours later."