The Talmud teaches that one is permitted to speak loshon hora about baalei machlokes, men of strife, who are at the forefront of a community feud. The purpose of speaking out against such individuals is to encourage other members of the community to dissociate themselves from them. Isolating instigators of a dispute can help restore tranquility to a community torn by strife. The undisputed rav of a community could, for example, urge his constituents to refrain from conversing with certain individuals, so as not to give them an opportunity to develop a following to create strife.

However, disparagement of one party in a dispute by the other usually results in mudslinging which further escalates the dispute. That is not at all what the Sages had in mind when they permitted speaking against baalei machlokes.

Sefer Chofetz Chaim, 19 Kislev, page 206

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