Shaikh Rabee bin Haadi Al-Madkhali [may Allaah preserve him] stated:

If a scholar-a criticiser- disparages a person who is deserving of disparagement and warns against him due to his bidah, he [i.e. this scholar is considered to be] from the people of justice – those who are sincere to the religion of Islaam and the Muslims and he is not an oppressor; rather he is a person who has fulfilled an obligation. If he keeps quiet about someone who deserves to be disparaged and warned against, he is one [guilty of] betrayal. If he remains silent to the extent that he defends and shelters bidah and its people, then indeed he has destroyed himself, dragged those who listen to him into a mess and active in aiding falsehood and rejecting truth.

[Source: Al-Mahajjah Al-Baydaa Fee Himaayah As-Sunnah Al-Gharraa Min Zallaat Ahl Al-Akhtaa Wa- Zaighi Ahl Al-Ahwaa’ page: 31. Slightly paraphrased]