Hilchos Lashon Hara chapter 4, section 9
A person that hold bad personality traits, for example, that he is haughty or is quick to fly into rage. Although we can see that he got used to these traits and his heart is not embittered because of them, it is forbidden to denigrate him and talk about aspects of his personality that are shameful. Because truthfully speaking, we often see people, even men who are learned in Torah, who don’t consider those bad character traits as being a significant sin, and are not aware to the severity of this type of behavior. Perhaps even the sinner is not aware to the severity of this bad behavior, and if he understood the seriousness of the sin that he is committing, perhaps he would make a strong effort with all of his might not to laps back into bad character traits. If one sees this person routinely expressing one of these bad character traits, one is obligated to reprimand him and explain to him the severity of the sin that he is committing. By doing so, one fulfill the mitzvah of “Reprimanding your fellow Jew” and it is possible that this person will concede to him that he was committing a sin. But at this moment this person thinks his behavior is justified and correct. Therefore it is forbidden to go and talk about him to other people.