Sefer Chofetz Chaim chapter 7 Halacha 5

There is a time when you are allowed to believe lashon hara as truth in a situation where rumors spread by numerous people which don’t stop for at least a day and half across the city that someone who is known to be bad because he does a sin out of spite of Hashem that every Jew knows is a sin, like adultery, murder, robbery and the like and now there’s rumors he did it again or did something else wrong. For example, if a person is known to be an robner and rumors go around the city that he did it again or that this time he killed someone, then people are allowed to believe and even take action against him like insulting him and speaking bad about him and his family like if a child was born out of wedlock with a married woman. These rumors can only be believed if they were started by multiple people and they were not people who are known to hate him. Because if they are his enemy then we don’t believe rumors that they start even the rumors don’t stop, like it goes viral over the internet, still they are not believed if started by known enemies, those that dislike him because it just might not be true. However, if random people or those who know him respectfully start talking about crimes he has done and the rumors continue then it’s permissible to believe them, repeat them and act upon them. The reason for this is because the Torah says you can’t speak lashon hara which would include accepting it only upon those Jews who are considered part of “your nation” but if they decided to remove the Yoke of Heaven and do these outlandish crimes then they aren’t part of “your nation” anymore. They are still Jews but halachas don’t apply to them like not speaking lashon hara and you don’t have to rebuke them because there is only a mitzvah to rebuke those part of “your nation” but if they decided to rebel then you shouldn’t rebuke them because they won’t listen at the verse in Proverbs says: “Don’t rebuke a scoffer.”