Shemiras Halashon Halachos
Hilchos Lashon Hara chapter 6, section 7 . Just as one may not believe lashon hara, so too, even in a case where one knows the story is true but the speaker does not take possibly alleviating factors into account and we know that it is a mitzvah to judge a person charitably (as stated in Shavuos 30a and this is a positive mitzvah according to many authorities) and the listener agrees to the speaker's version of what happened, he not only transgresses, You shall judge your fellow charitably, but is also considered a believer of lashon hara for accepting the worse version of the story.
Hilchos Lashon Hara chapter 6, section 8 . This applies if the person spoken of was an average person who generally does not sin but sometimes does. How much more if the story involved a God fearing person to whom the positive mitzvah of You shall judge your fellow charitably applies even more (as explained in the Rambam, Avos 1, and Rabenu Yonah, maamar 218).