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

es