Not very long ago a spy was arrested as a double agent. In describing the damage he did to the country, they said that many other spies were given away by him and executed.
The Chofetz Chaim (Shmiras HaLashon 2:4) says that when someone speaks Lashon Hara, Bais Din Shel Matah cannot punish him. That doesn't mean he caused no damage. In Bais Din Shel Ma'ala, the Satan prosecutes Am Yisroel. Who knows how much poverty and how many deaths came to the world because of him, asks the Chofetz Chaim? Although he has only caused damage indirectly, in Shamayim you are held accountable for all the indirect damage you cause. Don't be a murderer and plunderer!
"Zachor Eis Asher Asa Hashem L'Miriam BaDerech B'Tzaischem MiMitzrayim"
The Ramban and other Rishonim count, among the Mitzvos Aseh Min HaTorah, the mitzva of remembering and saying with your mouth each day what Hashem did to Miriam when she spoke about her brother Moshe. The Chofetz Chaim in Shaar Tvunah Perek 12 says that being Mikayem this mitzva can save you from the aveira of Lashon Hara. More than just saying the pasuk, says the Chofetz Chaim, one must think about the incident in order for it to have its intended effect.
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