Sefer Chofetz Chaim chapter 6, halacha 7

The Chofetz Chaim says that there is a prohibition of accepting lashon hara not only if what the person is saying might be a lie, but even if what he is saying you know is true and the person saying it is giving a negative twist to it, if the listener does not judge the situation favorably when he is supposed to, not only does he transgress judging a fellow Jew favorably which some poskin say is a mitzvah in the Torah, “bitzedek tishpot amisecha” but he also received the prohibition of accepting lashon hara just because he did not judge the situation favorably. One of the examples I made up was what if you saw a hunter shoot a child in the forest and someone else ran over to you who saw the same thing and said this hunter should be caught and executed for this horrible act of viciously gunning down an innocent child. This is obviously lashon hara, but you yourself saw it happen however you have to judge the situation. What’s the child doing in a forest where people hunt? Did the hunter aim for the child or was he aiming for a deer, etc. etc.? You can’t come to conclusions so quickly and accept that the shooter is malicious, you have to judge favorably until all the circumstances are clear.

The Chofetz Chaim brought 3 proofs that not judging favorably and accepting lashon hara even if true is a sin:
A. Clearly the person who spoke the lashon hara sinned, therefore so to the one who accepted it.
B. Yeravam ben Navat was rewarded kingship over the ten tribes of Israel for not accepting true lashon hara from the prophet Amos of someone in Israel stabbing him, and he accepted that even if it is true it is a prophecy from G-d. So because he judged the prophecy positively though it was negative and against him he was rewarded to become king even though he became a very evil king.
 C. King Shaul actually accepted the lashon hara from Doeg that Achimelech helped David, which Achimelech admitted to but Shaul believed Doeg and killed many people for helping David because King Shaul thought David was rebelling against his rulership and wanted to dethrone him in his lifetime. Doeg was called someone who always speaks lashon hara because of this and King Shaul was branded someone who accepts lashon hara, even though it was true that Achimelech helped David but he didn’t judge him favorably that it was intent to help someone make a rebellion against his kingship.