It is not about having sympathy for the murderers to want to look into the reasons why these boys may have acted as they did. It's about wanting to understand so that, God forbid, if another child ever has the same kind of tendencies, normal and decent people might have a chance of intervening before it is too late.
As I understand it is fairly common for seriously disturbed children to act in a similar way towards animals, something smaller and more vulnerable than themselves which most children have developed empathy for. Some children, through experience/abuse or simply a form of mental illness do not develop this normal sense of empathy and this is a dangerous combination. Children experiment, that's how they understand the world, and they don't process long term consequences like adults do. Empathy is usually what prevents children from experimenting in ways which cause hurt or distress, and most children by 10 understand death well enough to be horrified by it and afraid of causing it, much more so than younger children who have no sense of danger. It's quite likely these boys understood what their actions were causing in terms of distress and likely death - where was their horror, empathy and fear? It is as though through punishment we want to make them feel those things retrospectively, even if only in the sense of fear and horror for themselves, but I don't know if that's actually helpful in preventing future cases, because they almost certainly wouldn't have had the foresight to understand that their actions would result in prison for themselves.
It's difficult. They absolutely did need to be removed from the public sphere in order to protect other children. I can see a sense of retribution too, but in terms of preventing future incidents of this type I think the causes should have been identified further. Yes absolutely investigate the parents, and prosecute if they were found to have been at fault in terms of abuse or neglect. Not that parents are always responsible for their children's actions, but when abuse and negligence has caused a child to lack empathy in such an extreme way, that is horrendous, and the people responsible do need to be held to account.
It does seem very possible that Venables in particular may never be able to be rehabilitated. Whether that is down to early childhood experiences or a kind of personality disorder is not something any layperson can say.
It's my belief that the phenomenon of disturbed children hurting and killing animals does have strong parallels, just much less reported, and should be taken more seriously and looked at as a phenomenon. It is already known to be one of that he signs a child may grow up to become an adult who will murder, however, some children who torture animals do grow up and become 'normal' for want of a better world. I believe that to these two boys it might well have seemed no different or possibly obly one step higher to choose a young child instead of an animal for their entertainment :(