I just looked it up:
The sources for the above halachos is Chafetz Chaim, Hilchos Loshon Hora, Klal 6, Halachos 10 & 11. Also see the B’eer Mayim Chaim there, footnote #30, especially the second to last and third to last paragraphs.
The Chofetz Chaim there speaks very shtarkly about how if your life or someone else’s life is in danger or someone may be harmed, you are only allowed to be “choshesh”. But you are not allowed to believe it. Not only that , but you are not even allowed to consider it a “safeik”! All you are allowed to do is to protect yourself.
He also says that it is forbidden to relate to the person ANY differently, even in your heart!
In the Be’er Mayim Chaim, the Chofetz Chaim writes that even though you are allowed to give over the information when there is a possibility of someone being harmed, this is only in a case where you know that the listeners will not treat the subject any differently.
He writes there that if they will accept his words as truth and will embarrass them because of it, he is NOT allowed to tell them the l”h! This is despite the fact that someone is in danger of היזק from the subject!