It is permissible to speak disparagingly concerning a rasha, wicked individual.
With regard to potentially harmful speech, it is obvious that one may not say anything that could cause harm to any person unless the Torah permits harming him. Just as it is forbidden to steal from a sinner, so too it is forbidden to speak about him in a way that could cause him financial loss. If, however, the potential harm is in the form of embarrassment, it would not be forbidden, if the person is one whom we are permitted to embarrass according to halachah.
Sefer Chofetz Chaim, 25 Tishrei, page 98