Laws of Lashon Hara / 8: 8-9 (66)

8. Some say that it is permitted to speak lashon hara about a baal machlokes -one who is at the forefront of a feud – when these prerequisites are met: A. His speaking up will cause an end to the dispute. B. He knows firsthand that he is the baal machlokes. C. His intention is for a constructive purpose. D. There is no other way to put an end to the feud.

9. There is a rule and excommunication from our ancestors against slandering the dead, even one who is considered to be an ignoramus, how much more so about a Torah scholar, and how much more so regarding his words of Torah.