What would you have said to Stefan Kiszko's mother Charlotte after you'd had him strung up?
You're comparing apples and oranges.
The people Kay mentioned, along with Venables, were/are all extreme cases, in which no amount of rehabilitation and prison time will ever do anyone any good.
Stefan Kiszcko always claimed he was innocent, even after the Home Office offered him a chance of parole if he admitted the murder of Lesley Molseed.
Venables, Hindley, Brady & Sutcliffe are a completely different kettle of fish, where their continued danger to the public was/is well documented