An obvious case where conveying negative information is required is where a person is overheard plotting to harm someone. While we have seen that when a Jew may be in danger, even secondhand information can be related, it is imperative that one first determine that the plot is apparently serious and not a bad joke.
The condition that one first reprove the person against whom he intends to speak applies only if one believes he can change the alleged plotter’s attitude. Otherwise, warning him would only induce him to modify the plot to ensure that it not be uncovered.
The remaining conditions are required here no less than in an ordinary case of constructive speech: One’s intent must be solely to avert harm (and not, for example, for the pleasure of thwarting the plotter); if an alternative exists – including not revealing the identity of the plotter – then that avenue must e pursued. Also, one must be reasonably sure that the plotter will not be caused undue harm. If the potential victim is known to be hot-tempered and can be expected to take revenge, he cannot be spoken to. Alternative means would have to be found to protect the victim without endangering the plotter. (The case of one who overhears a plotter will be discussed in greater detail in the laws of rechilus).
Sefer Chofetz Chaim, 12 Kislev, Page 192