Scottish born Dennis Nilsen, also known as 'The Muswell Hill Murderer' and 'The Kindly Killer' is the UK's third most prolific known serial killer, known to have killed at least 15 men and boys between 1978 and 1983. Nilsen was born in Aberdeenshire to a Scottish mother and a Norwegian father, however his parents divorced whilst Nilsen was just 4 years old; probably as a result of his fathers alcoholism. His mother remarried but showed an apparent unwillingness to look after her son, sending him to live with his grandparents; however his grandparents sent Dennis back to live with his mother after just a couple of years. Nilsen, like many serial killers, claim a traumatic childhood; and he has cited his first disturbing event as when his grandfather died. His mother was a strict Roman Catholic and would often lecture his son on the 'impurities of flesh'.

Nilsen left school aged 16 in 1961 and enlisted straight into the British Army as a cook, with whom he served in South Yemen, Cyprus, Berlin, and the Shetland Islands. Nilsen left the army 11 years later in 1972, aged 27, and served for a short period as a police officer before taking up employment as a civil servant working in a Jobcentre. Nilsen was a very professional and straight seeming man, even having heavy involvement in trade unions and joining in on the picket lines various industries. Nilsen's murders were first discovered by well known drain cleaning company Dyno-Rod, who had responded to a call reporting a blocked drain in the block of flats that Nilsen lived in at Cranley Gardens, Muswell Hill. The worker who had attended the call found that it was blocked with "flesh-like substance", and he reported this to his supervisor. The next day the worker and his supervisor attended the scene to assess the drain, before finding that it had since been unblocked; this made them suspicious and so they decided to inform the police as a precautionary measure.

The police responded to the complaint and found some small bones and what looked like chicken in a pipe which led off from the drain, they took samples and it was confirmed by forensics that these were of human origin. Three policemen, including Detective Chief Inspector Peter Jay, attended the scene and awaited the return of Nilsen from work. Upon Nilsen's return, DCI Jay introduced himself and explained that they had come about his drains. They asked Nilsen where the rest of the body was hidden, to which Nilsen calmly informed them that they were hidden in plastic bags in the wardrobe. He was immediately arrested on suspicion of murder and taken back to a police station, on the way back to the station Nilsen admitted that he had killed "15 or 16". Upon a subsequent search of his flat, they found a third body hidden in a large tea chest. They later searched his former address at 195 Melrose Avenue, Cricklewood. At Melrose Avenue he had burnt his victims bodies on a bonfire.

Nilsen admitted to killing 15 men and boys, most of whom were students or homeless men who he had picked up in bars and invited back to his house. He claimed that he strangled and drowned all of his victims during the night, before waking up with little memory of what he had done. He used skills gained during his time in the British Army to help dispose of the bodies. He would often keep the bodies in different locations in his home for several months before dismembering them. It was not until he moved from 195 Melrose Avenue, Cricklewood, to his Cranley Gardens flat that he had trouble disposing of bodies and he would store them in bin bags in his wardrobe. Neighbours had begun to notice the smell of rotting flesh in the days leading up to his arrest. It was eventually discovered that Nilsen had murdered three people at this address, and had begun attempting to dispose of the bodies by boiling their hands, heads and feet to remove flesh before chopping it up into small pieces and flushing them down the toilet, it was by doing so that he blocked drains and was subsequently caught out.

Just 8 of the 15 Nilsen victims have been identified, but it is known that the murders took place between 30th December 1978 and 26th January 1983. The first of Nilsen's known murders was of Stephen Dean Holmes, who Nilsen claimed he had met in a gay bar before taking him back to Melrose Avenue where he strangled him with a necktie until he was unconscious, and proceeding to drown him in a bucket of water. It was not until 12th January 2006, 27 years after the event, that it was announced that the first victim had been identified. Stephen Dean Holmes was actually just 14 years old, and had gone missing on his way back home from a concert that he had attended with friends. On 9th November 2006 Nilsen finally confessed to the killing of Holmes in a letter sent from his cell to London newspaper the Evening Standard. The Crown Prosecution Service decided that a prosecution would not be in the public interest, Nilsen was after all already serving a full life sentence.

Following the first murder, Nilsen attempted to murder a student from Hong Kong who he had met in a public house. Although the police question the student, Andrew Ho, he decided not to press charges. Nilsen was released without charge, and was free to murder for the second time. The second known murder occured almost a year after the first on 3rd December 1979, when 23 year old Canadian tourist Kenneth Ockenden met Nilsen in a pub and subsequently escorted his victim on a tour of Central London; Nilsen then offered Ockenden the opportunity for another drink at his home. Nilsen strangled Ockenden with the cord of a pair of headphones whilst Ockenden was listening to a record. The murders became a more frequent occurrence, with murder number three occuring in May 1980, this time it was 16 year old Martyn Duffey - a 16 year old runaway from Birkenhead. Duffey, like Ockenden, accepted an invite back to Nilsen's home; Nilsen was strangled and drowned in a kitchen sink. Victim 4 was Billy Sutherland, a male father of one from Scotland who had been working as a prostitute, who was strangled by Nilsen's own hands.

Victims 5, 6, 7, 8, 9, 10 and 11 remain unidentified by Nilsen, who recalls certain characteristics about them but has been unable (or possibly unwilling) to assist police with their identification. At some point between these 7 unidentified murders, another attempted murder by Nilsen was reported. This time a Scottish barman, Douglas Stewart, woke up being strangled by Nilsen and was able to fight Nilsen off. He called police immediately, however the officers took no action who brushed it off as a domestic argument. This was the second missed opportunity to take Nilsen off of the streets. Victim number 12 was a man named Malcolm Barlow, who was murdered on 18th September 1981. Nilsen found him slumped in a doorway near his home, and in a rare act of compassion took him in and called an ambulance for him. When Barlow was released from hospital the following day, he returned to Nilsen's home to thank him and was invited in for a meal and some drinks; he was murdered on that same night. The Barlow murder was the last murder in Nilsen's former address.

Nilsen moved to his second address in November 1981, and there are two known attempted murders during this period, although both of these only came to light after his arrest was publicised. John Howlett became murder victim number 13 in December 1981 and was the first known victim at Nilsen's new address. He had initially fought back against Nilsen and was at one point was actually strangling Nilsen, however Nilsen eventually drowned him after holding his head under water for five minutes. This body was one of the three to be found in the Cranley Gardens flat. The next Cranley Gardens victim was Graham Allen, a homeless man originally from Scotland, who was invited back to Nilsen's flat for food. He was strangled whilst eating an omelette that Nilsen had prepared for him, and then left in the bath for 3 days before Nilsen eventually dismembered him. The 15th and final murder was that of Stephen Sinclair, a 20 year old Heroin addict who Nilsen had met in Oxford Street, on 26th January 1983. Sinclair managed to beg a hamburger off of Nilsen, who then suggested that they go back to his flat. Sinclair was plied with alcohol which, when mixed with his drug intake, left him in a stupor. He was strangled and, like victims 13 and 14, his body dismembered. It was the flesh of Stephen Sinclair which had been found by Dyna Rod in the blocked drains less than two weeks later.

Nilsen pleaded 'Diminished Responsibility' as his defence, in an attempt to seek a guilty plea for manslaughter. This defence was unsuccessful and he was convicted of six murders and two attempted murders; he was sentenced to life in prison on 4th November 1983, with the Home Secretary imposing a 'whole life tariff' meaning that he would never be released; although he was granted permission in 1993 to give a televised interview from prison. Nilsen is still in prison now, currently at HMP Full Sutton, and is fighting a legal battle to be allowed to publish an autobiography entitled 'The History of a Drowning Boy'.

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