Kester David, 53, from Wood Green, was found badly burned under a bridge in Palmers Green in 2010.
Police made a fresh appeal for information about his death last year.
On Monday the police said they had closed the reinvestigation and that the Met commissioner had met Mr David's family to apologise personally.
Sir Bernard Hogan-Howe said: "There were obviously some things that should have been done by those officers, which quite simply were not.
"It is also completely unacceptable that a grieving family were treated with such insensitivity."
His family have always maintained he was killed for working as a police informant and have claimed police failed to look at CCTV or speak to witnesses promptly.
The initial investigation by Scotland Yard into Mr Kester's death concluded he committed suicide.
A year later an internal investigation, led by inspector Brian Casson, found a "catalogue of errors" and a "failing in duty" as full CCTV and mobile phone records were not checked and witnesses were not interviewed.
Sir Bernard insisted that the reinvestigation, now closed by the Met, was "thorough and explored every single line of enquiry open to us".
Detectives concluded there was no evidence or information that a third party was involved in his death.
Despite a number of appeals during the reinvestigation made to local and national media and on BBC Crimewatch, no arrests were ever made in connection with his death.

The troubled NHS trust has been struggling financially and has some of the worst response times in England.
However, the Care Quality Commission (CQC) said staff were committed to providing high quality, safe care despite suffering from low morale.
The health watchdog said inspectors also saw some "outstanding practice".
Prof Sir Mike Richards, the CQC's chief inspector of hospitals, said EMAS, which serves about 4.8m people across six counties, required improvement.
He said: "There was an unrelenting demand for emergency services combined with a lack of staff and resources to meet the need.
"We found that while people were cared for and treated well, there were insufficient staff and a lack of appropriate skill-mix to meet the needs of patients in a timely manner."
The trust was told it must ensure there are sufficient emergency vehicles to safely meet demand and response times must also meet the needs of patients by reaching national targets.
In April it was revealed EMAS had overspent by almost Â£12m in the last financial year and had to take out an additional loan to help with running costs.
Staff were also reported to be under pressure and unable to take scheduled breaks.
Richard Henderson, acting chief executive of EMAS, said: "We will be continuing our recruitment campaigns. Last year we recruited over 300 staff. This year again we will be doing exactly the same.
"We will be increasing the numbers of vehicles available but most importantly we need to ensure we retain our existing workforce."
EMAS said emergency calls had increased and handover delays at hospitals had caused it to incur the extra costs.
The service which covers Derbyshire, Leicestershire, Lincolnshire, Northamptonshire, Nottinghamshire and Rutland has about 2,700 members of staff and receives approximately 2,000 calls a day.

Steve Sankey was elected in the East Mainland, South Ronaldsay and Burray ward of Orkney Islands Council.
Party co-leader Patrick Harvie tweeted that he was "delighted" with the first Green gain in the elections, and the party's first Orkney councillor.
Robbie McGregor won the uncontested Shetland South seat for the SNP.
He is the first councillor to represent the party on Shetland Islands Council.
The Greens said Mr Sankey was the first member of a mainstream national party to be elected to Orkney Islands Council, where candidates have traditionally stood as independents.
Both Orkney and Shetland councils were independent holds.
Scotland Yard has apologised to the family of a police informant for "shortcomings" in the original investigation into his death.
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A report into East Midlands Ambulance Service (EMAS) has found it has "insufficient staff" to meet the needs of patients in a "timely manner".
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Orkney has elected its first ever Scottish Greens councillor, while the SNP has gained its first representative in Shetland.