The problem with vigilante justice in games is that that gives the thief or whatever the upper hand. It's quite obvious that you violently attacked him and stole all his stuff....but how can you prove that he stole something really valuable of yours? Thus, you have to leave it in the hands of the authorities (be it game makers or police) to produce justice.....which is not always the best option. Of course, it is much better than being persuceted yourself....
As far as mob rule goes....that's just sad. Seems to me that the makers of these games need to pay more attention to a justice system.....not nescessarily staff, but voulenteer police. This has worked in many text-based ORPGs that I know of. They're players, for the most part, but when needed they can act as representatives of the game makers...not to dole out punishment, but to basically play detective. Find out what happened, get names, time, evidence, and make a recomendation to the game administrators.