Dennis Nilsen terrorized London in the late 70's and early 80's, killing 15 men over a five-year period. Nilsen would pick up young men at bars and take them back to his home. There, he'd strangle or drown them. Once dead, he'd bathe and dress the bodies, according to The International Business Times.
Nilsen became increasingly disturbed by his sexual encounters, which only seemed to reinforce his loneliness when they were over. He met his first young victim in a pub on December 29, 1978, and invited him home, as he had on previous occasions. The next morning, overcome by a desire to prevent the young man from leaving, he strangled him with a tie before drowning him in a bucket of water. Taking the corpse to his bathroom to wash it, he then placed it back in his bed, later remarking that he found the corpse beautiful. He attempted to have sex, unsuccessfully, then spent the night sleeping next to the dead man. He finally hid the corpse under his floorboards for seven months, before removing it and burning the decaying remains in his back garden.
Nilsen had another close call with the police in October 1979, when a young student accused Nilsen of trying to strangle him during a bondage-play session. Despite the student's claims, no charges were pressed against Nilsen.
Nilsen encountered his second victim, Canadian tourist Kenneth Ockendon, at a pub on December 3, 1979. Following a day of sightseeing and drinking, which ended at Nilsen's apartment, Nilsen again succumbed to his fears of abandonment and strangled Ockenden to death with an electrical cable. He cleaned up the corpse as he did before, and shared the bed with it overnight. He took photos, engaged in sex and finally deposited the corpse under the floorboards, removing it frequently and engaging in conversation, as if Ockenden were still alive.
His third victim, some five months later, was Martyn Duffey, a homeless sixteen year old, who he invited to spend the night on May 13, 1980. As with his first victim, Nilsen strangled then drowned him, before bringing him back to bed and masturbating over the teenager's corpse. Duffey was kept in a wardrobe for two weeks, before joining Ockenden under the floorboards.
His next victim was prostitute Billy Sutherland, 27, who had the misfortune of following Nilsen home one night. He too was strangled. Another one of his victims, 24-year-old Malcolm Barlow, was an orphan with learning disabilities, who was soon dispatched by strangulation.
By 1981, Nilsen had killed 12 men in the apartment, of whom only four could be identified. Given his penchant for preying on the homeless and the unemployed in a large city, this is probably less surprising than it might be in a smaller community.
He admitted to police that he kept the bodies for extended periods of time before disposing of them so that he could have sex with them and talk to them. He is currently serving a life sentence at the HMP Full Sutton prison in Yorkshire, England.