Ian Brady and Myra Hindley aka The Moors Murderers who abducted, assaulted and killed at least four children between 1963 and 64 and were finally caught when Hindley's brother-in-law saw Brady killing 17 year-old Edward Evans.
Sadly not the most disturbing story I know but one of the first I ever read when young and which set me off on a life-long fascination with such people.
http://criminalminds.wikia.com/wiki/...d_Myra_Hindley