The verse "The Lord will cut off all flattering lips" (Psalms 12:4) is explained by Rabbi Yonatan in the name of Bar Kappara as referring to lashon hara (evil speech). He states that lashon hara is as severe as idol worship, sexual immorality, and murder. The seriousness of idol worship is demonstrated by Exodus 32:31, which describes the people's sin as a great sin. Sexual immorality is condemned in Genesis 39:9, where Joseph asks, "How then can I do this great wickedness?"
Murder is considered a great sin, as expressed in 2 Samuel 4:13, which states, "A great guilt has fallen upon us." Lashon hara is referred to as "great speaking" (Proverbs 18:21) and it is written that those who speak lashon hara say, "We will increase our tongue" (Psalms 12:5). Rabbi Yosei bar Hanina teaches that no one speaks lashon hara about their friend unless they deny the fundamental principle that "Who is the Lord, that we should obey him?" (Exodus 5:2).
The psalmist also cries out against those who speak lashon hara in Psalm 94:16, asking, "Who will rise up for me against the wicked?" The punishment for those who speak lashon hara is severe, as even hell itself cries out that it cannot bear their presence. God responds to this by saying, "I am above and you are below" (Psalms 50:4), meaning that the punishment for lashon hara is worse than that of a sword or arrow.
A sword cannot kill unless it is close to the target, but an arrow can be shot from a distance and hit its target anywhere (Jeremiah 9:7, Psalms 120:4). The phrase "if you have turned your tongue against your fellow" (Job 6:30) implies that speaking lashon hara is equivalent to shedding blood, as it is taught by Rabbi Shimon ben Yochai that lashon hara kills three people: the speaker, the listener, and the one about whom it is spoken.
Rabbi Bibi explains that we find that Saul killed four people in his lifetime, but he received a severe punishment for not protesting when others spoke lashon hara (Yoma 22b). Even the High Priestly city of Nov was destroyed because the people spoke lashon hara (Sanhedrin 104b). Rabbi Zera sent a message to Rabbi Simon asking why he did not protest when he heard lashon hara. Rabbi Simon replied that he was only among those who sighed and groaned.
Rabbi Zera asked what would become of those who sighed and groaned, and Rabbi Simon replied that they would be punished as well (Ketubot 5b).