Moses then reviewed with the Jewish people the laws regarding divorce, exemptions from military service, conduct when taking collateral on a loan, kidnapping, and the skin disease known as tzara’at. This disease afflicts mainly people guilty of improper speech, just as Moses’ sister Miriam was struck with it when she voiced her disapproval of Moses’ behavior in divorcing his wife Tziporah.
The Power of Words
זָכוֹר אֵת אֲשֶׁר עָשָׂה ה' אֱלֹקֶיךָ לְמִרְיָם וגו':
(דברים כד:ט)
[Moses told the Jewish people,] “Remember what G‑d did to Miriam.”
Deuteronomy 24:9
It is important to realize that Miriam did not lie or even criticize her brother, Moses. She merely voiced her disapproval of his behavior without bothering to discuss her grievances with him privately first. Nonetheless, Miriam was immediately punished for having spoken against Moses.
This teaches us how careful we must always be when we talk (or write) about other people. Even seemingly harmless discussion can easily slip into gossip or slander. Rather, we should always be careful to speak and write constructively. If someone’s behavior seems inappropriate, we should clarify matters with them privately, thereby sparing everyone the grief that inevitably results from misunderstanding.1
And how can you deal with people who have narcissistic personality disorder or narcissistic traits? Do you have to speak properly about them even if you see how they secretly undermine other people's life and in what ways they control them?
I believe you are allowed to warn others if you have heard that they are about to engage in business with a proven crook to avoid them suffering financial loss. So maybe the same applies with other behaviour. The only problem, though, it is difficult to prove that someone is a narcissist even if you know they harm others, especially 'secretly'. If it were me, I would say nothing if someone asked me about them or say 'I prefer not to say' and let them make their minds up. Also, would we be allowed to divulge this information with regard to a shidduch to avoid a lifetime of misery?