2. Mary Bell
Found guilty of killing 2 boys in different occasions. Bell was convicted for murder at the age of 11. The first murder caused the death of a four year a old boy who was strangled to death. She left notes in a different location of the murder with the help of her friend Norma Joyce Bell, saying that she was responsible for the killing. But the police at the time thought the notes were a prank.
The second murder, another boy age 3 was killed by strangulation with the friends help. This time a letter N which was finished into an M was carved with different hands on the boys stomach. With that Norma was acquitted while Mary served 12 years in prison.
The last known news about her was that she became a grandmother in 2009.