I can see the importance of not speaking lashon hara/slander but I think that this emphasis tends to diminish the equal importance of warning the public or individuals about dangerous friends or groups. There must be many more sources for this but the two that I recall off hand are Kamtza and Bar Kamtza by which the Second Temple was destroyed. Not so much for some individual transgression but the fact that no one objected. Also the events surrounding the concubine at Giva. It is not at all that some people in that one city had killed the concubine, but rather that no one in that city nor in the entire tribe of Binyamin objected.
See sefer hamidot [of Rav Nahman] the section on embarrassment. he brings there a statement from the sages that it is permissible to embarrass the religious authorities that make money off of the Torah [and what religious authorities don't?], and one must not stand before them, and the clothing they wear is like a saddle on a donkey. So we see that sometimes one must object. Certainly the Gra and Rav Shach objected to great evil, even though it is clear that no one paid any attention to what they were saying until this very day.
