So we take two children, with impaired understanding of life, so educationally limited, and make them live almost as long as their lives to the date of their admittedly horric crime, knowiung they will be executed at the age of 18.
This may make sense in a country where people can be kept waiting on Death Row for years, and then be executed when evidence exists that they were innocent because of due court process having been followed and it cannot be overruled.
This may equally make sense to someone with a similar lack of empathy to the actors in this case. Those eight years at that age are a very long time.
That rehabilitation failed is more an indictment of the way they were dealt with in prison. We don't know that enough was done, or tried to be done. If that story about the warder is true, then adequate car4e was not taken. How did Venables become able to download child pornography? These are people who have been failed first by their parents, and then by society. Venables should certainly be kept out of the way of doing harm.
Maybe some people do not deserve a second chance, but the choice of whether it is offered should not be in the hands of people with a shortage of empathy themselves. Empathy, BTW, is not a synonym for sympathy, but an understanding of others' minds. It can, itself, be found in nasty people, who use their understanding to hurt.