If a resident of a city is an established sinner, violating many laws, it is only permissible to speak Lashon Hara about him under certain circumastances!
Here the Chafetz Chaim gives reference, enumerating the conditions that must be fulfilled to speak Lashon Hara in this case.
1. The speaker must have constructive intentions to encourage others to avoid the ways of evil, once they realize that such behavior is disparaged, and perhaps even to inspire the sinner himself to repent. He should not intend to benefit from exposing the other's faults, nor should he speak out of hatred.
2. The speaker may not exaggerate about the subject's behavior in any way.
3. The speaker also should not act deceitfully, disparaging the person privately behind his back, yet flattering him in his presence; he should be comfortable speaking about him publicly.
If, however he has a specific fear of retribution or swishes to avoid a public controversy, he may relate it to individuals privately. The speaker must be certain that his intentions are to prevent others from learning from the sinner's evil ways.
Who is classified as an established sinner? One who is designated as such by the town elders, based on which there can be no doubt, due to constant, numerous reports regarding his adulterous acts or other transgressions that are of the type that everyone knows they are forbidden.
One may not relate information for their own personal satisfication!
Speaking Lashon Hara about an Established Sinner-from http://www.facebook.com/group.php?gid=7507446732 Stop Lashon Hara (gossip) Torah group