The Guild

Ken Follet was a normal average person, a widow with one married daughter. When her two kids are abducted, abused and killed the daughter tries to kill herself. Ken decides to take the law into his own hands. He waits six brain festering years for the only man ever punished for the children's deaths to be released from prison Ken has his revenge. Finding that he has a hidden liking for violence Ken decides that this is his way forward. By starting the Guild, he brings down a reign of terror on a small city. In a very short time, the city is controlled by the Guild and for some reason petty crime almost stops.
Can violence ever be a justified way of stopping violence? Would the police be able to act against a criminal group like the Guild? Would you be willing to go against the Guild if you found that they were working in your town?
Could it ever happen?