If a person who is not so righteous but not so bad, in the middle, most of the time doing the right thing and you know he did something really bad that everyone knows is a sin, for example he ate non-kosher food like pig or shellfish. Or even if he did a sin not so famous but you warned him not to do it and he did it anyway. You are forbidden to tell anyone what he did because it’s possible he did teshuva and truly regretted what he did. Only if he is constantly transgressing the sin the circumstance might be different and we’ll learn more about that in halacha 7. If you do reveal it to someone else it’s considered a various grievous sin of lashon hara if said behind his back and if said to his face in front of a crowd the person has embarrassed him and therefore might not have any share in the World To Come assuming the sinner did repent. The Chofetz Chaim elaborates in the Be’er Mayim Chaim note 14 that though there is a status in the Torah of someone you are allowed to hate if you saw he did a sin that does not mean you can speak lashon hara because he is still considered part of “your nation” so though he is not considered “your brother” which allows you to hate him as long as you don’t know for sure that he did teshuva since the Chofetz Chaim says that the main part of repentance is regret in one’s heart which only G-d really knows about. But you still have to rebuke anyone part of your nation nicely in private therefore you cannot embarrass and denigrate him in public. If a person does hear lashon hara of this sort he can be cautious but may not accept it as fact, even if two people would tell him unless convicted in official Jewish court.