Need to speak lashon harah

 Pischei Tshuva (O.C. 156): I want to note here that while all the books of mussar are greatly concerned about the sin of lashon harah, I am greatly concerned about the opposite problem. I want to protest about the even greater and more common sin of refraining from speaking negatively when it is necessary to save someone from being harmed. For example if you saw a person waiting in ambush to kill someone or breaking into someone’s house or store at night. Is it conceivable that you would refrain from notifying the intended victim to protect himself from the assailant - because of the prohibition of speaking lashon harah?  By not saying anything you commit the unbearable sin of transgressing the prohibition of Vayikra (19:16): Do not speak lashon harah [but] do not stand idly by when the blood of your fellow man is threatened? By not speaking up, you violate the mitzva of returning that which is lost to its owner Devarim (22:2). Now if you can understand the obvious necessity of speaking up in these cases then what is the difference between a robber breaking into someone’s house or store or seeing that his servants are secretly stealing from him or that his partner is deceiving him in their business or that another person is cheating him in commerce or that he is lending money to someone that you know doesn’t repay? How is this different from stopping a proposed marriage to someone you know is a wicked person who would be a horrible husband. Saving a person from these situations is clearly included in the command (Devarim 22:2) to return to the person himself or his money. From where do we get the mistaken idea that in the case of murder, I will speak up but that it is prohibited to say anything in other situations where someone is being harmed? The general principle is that these are matters which depend upon the speakers motivation. If the informant’s intent in relating these matters is entirely to cause harm that is lashon harah. However if his intent is to bring about benefit to the other person and to save him and to protect him – then it is a great mitzva. In my opinion this is the underlying intent of the Yerushalmi which the Magen Avraham brings which says that it is permitted to speak lashon harah about people who cause disputes. … It is obvious that even concerning those who cause disputes it is not permitted to speak lashon harah gratuitously about them in all matters. It is only permitted for those things directly related to the particular dispute. It is only permitted concerning that which they are trying to harm others. In such a case it is permitted to reveal degrading things about them in order to save others. … Unfortunately I have seen many times where someone witnesses another person trying to cause harm to someone – and he suppresses the information and says, “Why should I get involved in a matter which isn’t my business…However one needs to be very careful about these and similar matters. Our Sages have said – when the permissibility depends on motivation - it says, “And you should be afraid of your G‑d.”