The danger with vigilantism is that an innocent person is incorrectly perceived to be a criminal by the vigilante, and also that the vigilante's punishments are disproportionate. That's why we as a society have agreed that trial in front of a jury is the most reasonable option. That said, it becomes easier to sympathize with the romantic idea of vigilantism when corrupt media influences the jury pool, and corrupt courts either don't punish criminals, or let them off easy.