Shemiras Halashon Halachos
Hilchos Lashon Hara chapter 6, section 11 Even when one is permitted to suspect the person spoken of this is only to guard oneself from harm. G-d forbid to do any action or cause him any harm or evil. Even if a kosher witness testified against him in a beis din, this only helps if he swore to the story's veracity. Even more, even to hate him in one's heart is forbidden by the Torah and one certainly cannot free oneself from any obligations one has towards him.
Hilchos Lashon Hara chapter 6, section 12 If one transgressed and believed lashon hara regarding sins against God or man one should strive to remove the belief from one's heart, accept upon oneself to never again believe lashon hara, and confess the wrongdoing. This will repair the sins involved as explained in the introduction unless one already repeated the lashon hara to others.