The most common constructive purposes which justify speaking negatively about a person fall under four categories:
(1) To help the person about whom one is peaking (e.g. to discuss a person’s faults with someone else in order to help the person improve).
(2) To help those who are being adversely affected by the person’s behavior:
(a)providing help for people physically, psychologically or financially victimized by him;
(b)protecting people from falling victim to his behavior in the future; or
(c)warning the unsuspecting who are seriously considering entering into an unhealthy social or business relationship with him.
(3) To help put an end to a dispute in which the person is involved and which threatens to divide the community.
(4) To help others learn from the person’s mistakes.
Sefer Chofetz Chaim, 12 Cheshvan, page 132