Shemiras Halashon Halachos
Opening to Hilchos Lashon Hara and Rechilus (1) Through Hashem's love of His nation Yisroel and great desire for their benefit to the extent that He called them sons, and the portion of Hashem, and an inheritance, and many affectionate terms that indicate His great love for Yisroel as it says (Malachi 1:2), "I loved you, says Hashem," he distanced them from all bad character traits and especially from lashon hara and rechilus. For this would bring Jews to quarrels, and this can often lead to bloodshed as the Rambam writes in hilchos Dei'os (7:1): "Even though one receives no lashes for this negative command, it is a great sin and can lead to the killing of many souls in Yisroel." Therefore, it is juxtaposed to the verse that says (Vayikra 19:16), "Do not stand by your neighbor's blood." Go and see what happened to Do'eg the Edomi and to Nov the town of kohanim.
Furthermore, many great evils are caused by this disgusting trait. We know that the sin of the snake was principally through lashon hara when it spoke evil against Hashem saying, "From this tree He ate and created the world." Through this, it tempted Chava as Chazal (Shabbos 146a) say, "The snake came upon Chava and placed poison in her." Thus, it caused both immorality and death to the whole world. We see that there was bloodshed. Through this, it caused Adam and Chava to transgress the will of the Holy One. Thus, someone who talks lashon hara is utilizing this trait that destroys the world.
Also, the main reason Yisroel went down to Egypt was initially through this sin as it says (Bereishis 37:2), "Yosef brought evil report of them to their father." Through this, heaven decreed that he would be sold as a slave, measure for measure, for he had said that they called their brothers slaves.
Furthermore, the entire reason for our present exile is the sin of the spies as it says in Tehillim (106:26-27), "And He raised His hand against them to make them fall… and to scatter them in the lands," as Rashi explains there. So writes the Ramban in Chumash on the story of the spies (Bamidbar 14:1), and the Gemara says in Erchin (15a) that the principle sin of the spies was lashon hara, speaking evil of the land, and because they cried a weeping for no reason, weeping for generations was decreed upon them. Also, many countless evil happened to us through this severe sin, for all the sages killed by Yanai in the days of Shimon ben Shetach, King Yanai's brother-in-law, was also through rechilus. And the killing of the Tana R. Elazar Hamoda'i that led to the destruction of Beitar, was also through rechilus when they slandered him to Ben Koziva as it says in Midrash Eicha (Eicha Raba 2:4).