The Chofetz Chaim says that until now we were talking about a person who will regret the wrong he did, because his evil inclination just over cane him, maybe it will take a little force like the court, relatives, or his rabbi, to get him to realize his wrong doing and repent but if he is one who has knowingly and decisively decided to throw away the yoke of Heaven and purposefully does not keep Torah and mitzvos or is someone who on purpose without any remorse, repeatedly doing a sin knowing it is wrong and maybe if he was told to stop he totally ignores it. This type of person even if he is not known to others as being evil but you know his character, the Chofetz Chaim says you are allowed to tell other about the sin you saw him do even if that isn’t the sin he normally transgresses. Even if others don’t know yet that he is evil it might even be a mitzvah to publicize his evil and disposable ways and you shouldn’t worry that others will suspect you of slandering someone, in order to be sure no one will follow in his path, because if you are speaking for the sake of Heaven with sincerity then the truth comes out and people will believe you. This also avoids a chilul Hashem for two fold, one people won’t learn from this bad person’s actions and two of something bad happens to the sinner people won’t question Hashem why such a terrible thing happened to him since they’ll know he is evil. However people who listen to this report cannot decisively believe what he is saying unless it is known that this guy is evil, but they should be suspicious until they verify the matter.