I know this will sound controversial, but I don't believe that society's obligation to protect children ends even if said child has hurt or even murdered other children.
In almost all cases when a child or young adult has killed or tortured another child, like Robert Thompson and the teenage killers of Shanda Sharer they were all victims of abuse, frequently physical and sexual. This doesnt excuse their actions obviously but a particular type of environment creates children who kill.
Its why it disturbs me to see people call these children "evil", "scum", imply they were born evil or even threaten violence against them. I'm a moral absolutist in that I don't believe violence against children is ever acceptable, save for self-defence, even if said child has killed and tortured others.