Jon Venables and Robert Thompson:
In 1993, Jon Venables and Robert Thompson, both 10 years old, became the youngest convicted murderers in modern British history. While at a shopping mall, they lured and abducted a two-year-old boy, James Bulger, away from his mother. Bulger's body was found two days later on railroad tracks. His body was so mutilated that the autopsy could not even reveal his current cause of death.
Diane Downs:
In 1983, Diane Downs shot her three children and drove them to the hospital. One of her daughters was pronounced dead upon arrival, her other daughter suffered a stroke, and her son was paralyzed from the waist down. Diane claimed a "strange man" tried to steal her car and then shot at everyone. However, police later found her secret diary which "detailed her obsession with a married man who did not want kids." This ultimately led to her arrest. She was sentenced to life in prison, plus 50 years.
Dennis Nilsen:
Dennis Nilsen murdered six people in the late '70s and early '80s. He'd pick up young men at London bars and bring them back to his home where he'd strangle them. He'd also bathe and dress the corpses, often keeping the bodies for weeks or even months, and would also talk to them and have sex with them before ultimately burning or flushing the remains down the toilet. In fact, he was only caught because "a drain outside his home became blocked by the human remains he had tried to flush away."