As I understand it, the most popular theory of criminal punishment amongst criminal law scholars and philosophers is so-called 'negative retributivism'. The basic idea is this: the goals of punishment should be exclusively
forward-looking (rehabilitation, incapacitation, deterrence, compensation, restoration etc). However, these forward-looking goals have to be constrained by considerations of proportionality to avoid things like excessive punishment and punishment of the innocent (e.g. framing an innocent person for consequentialist reasons). Negative retributivism, in other words, is not concerned with dolling out the right amount of suffering to the guilty but about ensuring that nobody is made to suffer unnecessarily.
At the theoretical level, this approach makes sense to me. I personally have
strong retributivist impulses, but on considered reflection I realise that these impulses are unjustified and I certainly wouldn't want a criminal justice system based on them. As a social mammal I'm probably evolutionarily predisposed to want to see transgressors of social norms get their just desserts, but inflicting pain and suffering as an end in itself to somehow balance the scales of justice really isn't a plausible view when you think about it. As
littlebabyjesus put it, an eye for an eye makes the world blind. What we really want, or at least what we really ought to want, is accountability for wrong-doers, justice for victims, protection for the wider public and, where possible, reform of the offender (the latter is rather far-fetched in this case of course).
Obviously cases like this one are hard. What the murderers and their accomplices did is just so radically evil, so depraved, that it's perfectly natural to have a strong impulse that they should be made to suffer and maybe suffer terribly. But when you step a bit back from that impulse and think about it, it doesn't seem terribly attractive.