A man is ultimately accountable only to his conscience. To refrain from action against a criminal merely because the law forbids it is to submit oneself to a system that one knows to be flawed, to place its integrity above what one knows to be right or wrong. "Because the law says I must do nothing, I can do nothing" is an abrogation of personal responsibility, of the very sentience that charactises us as human beings. It's a power exchange that the state will fall over itself to accept.
"Due process" is not justice, it is merely the best means we have yet arrived at to ensure that justice is carried out, and there will always be exceptions. I can certainly envision situations in which I would act regardless of the state's wishes, just as there are situations in which I would act against the state. I am bound by no law, the law functions merely as a means to communicate norms, and to salve the consciences of those who enforce it that they merely play their part in the system, that they are not ultimately responsible for their actions. As with the uniform and authoritarian structure of military institutions, this systemic de-individualisation allows one to commit horrific acts free from one's conscience, serving ultimately to perpetuate human suffering.
Let's be careful before we come down too hard on those puffing out their chests and wishing for mob justice. Read just about ANY thread here where a crime is talked about (or, sometimes, not even a crime, but perceived misjustice), and many posters HERE take the same exact tone. Wishing for prison rape, violent murder, severe beatings, etc. The BBS can't exactly see the high road on this topic from where I'm standing.
... and?
Sure, we can say we're just talking shit, and don't really MEAN it, but that's what you're condemning, so...
So let it be condemned.
The hatred which characterises these predictable responses is abhorrent, but worse is the way in which responsibility for the acts advocated is deflected from oneself onto others. The "he'll be taken care of in prison" crowd
is the face of the faceless mob which, once the act is done, will disperse into pretensions of civility. If one really believes that a man should have his balls removed with a rusty knife, then step up and make it happen.