It's a mixed bag. I have no trouble with the Bernard Goetz's form of vigilantism ..and this is surely preferred to a mass of people watching as a rape occurs in the subway in "not getting involved", but the mistaken identity guy could have just as easily gotten killed.
This time it worked out, more or less. This didn't require the courage that preventing crime needs in intervention, where you have unity and consensus of civic responsibility to maintain the society's standards of right and wrong. This only required rage, a "there he is!", and numbers.
..but, that said, I'm glad that the mob caught the scum and left just enough of him functional to stand trial.