But everyone does have a right to be treated in accordance with the law. Even in the case of a murderer, e.g., an abortionist, he has a right to a fair, speedy, and lawful trial by a jury of his peers. One who "takes the law into his own hands" by killing him is in a real sense committing an injustice, both against the abortionist and also against society whose social coherence depends upon justice being administered through an orderly and hierarchical system. Therefore such an action, even if it provides some benefit to society, can hardly be considered laudable or unworthy of at least some punishment.
This also suffices for the case of Geoghan.
Geoghan had already been tried and convicted by the normal process.
What Druce did was to administer the actual justice a modern judge is no longer allowed to.
And I don't see where a jury trial is a universal human right. It is a right in Anglo-Saxon Common Law, but that merely addresses Anglo-Saxon nations. There is nothing necessarily unjust about other methods of trial being used.
Furthermore, the tacit approval of the authorities of extra-curricular punishment of child molestors in prison could be seen as a proper authorization of these actions occurring. If they didn't want it to happen, the Dahmer's and Geoghan's would be purposefully isolated from other prisoners. That they are not, even though it is well known that many felons have violent reactions towards child molestors, seems an approval of the punishment they mete out.