Law and Order: Criminal Intent ran an episode for the new season regarding vigilantism as the result of being placed on the sex offender registry. The RSO in the story was a person who, at 19, pled guilty to sex with a 15 year old, and placed on the sex registry, which led to his murder. Though the show was obviously inspired the the murder of two sex offenders in Maine a few years back, it illustrates perfectly the mentality of the vigilante groups. In fact, I would not be surprised if Tracy Golden, Marina Hammond, Chris Brocious, Barb Ireland, or Xavier von Erck himself ended up killing a former offender someday.
Anyways the show synopsis is below:
http://www.imdb.com/title/tt1031394/synopsis