Musar zu Tehillim 120:3

And this is the thing about the group of slanderers:
Our Rabbis, may their memory be blessed, said (Arakhin 15b), “Anyone who speaks evil speech is as though he denied a fundamental principle [of faith] - as it is stated (Psalms 12:5), ‘Who have said, “we will make our tongue mighty; our lips are with us; who is lord over us?”’” Therefore, they considered him as if he denied a fundamental principle. For he causes and brings about great damage and much evil to his fellows by making them foul in the eyes of people or by other ways of [causing them] loss. And it is not likely that a man would prepare a mechanism of destruction and damage that is more bitter than death without it benefitting himself monetarily, unless it is to have his [evil] impulse honor himself and remove the yoke of the Heavens from upon him - to remove ethical constraints - as it is stated in the Psalm, “when the Ziphites came and told Saul, etc., O God, arrogant men have risen against me; a band of ruthless men seek my life; they are not mindful of You.” (This is a scribal error, as there is some difference between the verse brought down by the author, may his memory be blessed, [and his source]. As [in the source], it is written, “For strangers have risen against me, etc.” And the verse that he wrote here is found in Psalms 86:14; but it needs to be corrected and replaced with the verse as it is written there in Psalms 54:5.) And our Rabbis, may their memory be blessed, explained (Midrash Tehillim 54) that they intended that Saul would bless them, as he said to them (I Samuel 23:21), “May you be blessed of the Lord for the compassion you have shown me!” But they did not place God in front of them - as it is written in His Torah (Deuteronomy 27:24), “Cursed be he who strikes down his neighbor in secret.” And it is written (Ecclesiastes 10:11), “no advantage is gained by the master of the tongue!” And it is stated about Doeg (Psalms 52:4-5), “Your tongue devises mischief… You prefer evil to good; the lie, to speaking truthfully. Selah.” And they explained in Midrash Tehillim 52, “What do you benefit and what do you gain when you say evil speech? Is it not that you did not need money, for you had [already] become wealthy - as it is stated about him (I Samuel 21:8), ‘Saul’s chief herdsman.’ It was only because you preferred the evil to the good, and ‘the lie, to speaking truthfully’ - as you have removed His yoke.” And it is stated (Proverbs 6:30), “A thief is not held in contempt for stealing [to fill his hunger]; and it is written after it (Proverbs 6:32), “He who commits adultery is devoid of sense, etc.” This is meaning to say that he is worse than a thief, for [the former] needs to “fill his hunger.” Our Rabbis, may their memory be blessed, said (Midrash Tehillim 120) that one involved in evil speech is worse than both of them. For he does a great sin without benefit, as it is stated (Psalms 120:3), “What can you profit, what can you gain, O deceitful tongue?”

Consider the greatness of their punishment. Because of the greatness of their sin, it is almost impossible for Gehinnom to cleanse them of it, as explained in Tanna d'bei Eliyahu 18: "Just as the wicked speak lashon hara, which ascends until the Throne of Glory, so, angels descend from the mouth of the Omnipotent One and take those evildoers and cast them into the [nethermost] depths of Gehinnom. At that time, Gehinnom responds and says to the Holy One Blessed be He: 'Master of the universe, I cannot punish them as much as they deserve to be punished, and the whole world cannot punish them enough. Did this speaker of lashon hara not sin from the earth until the heavens? But first send Your arrows from above, and then they will get from me coals of the broom bush from below, as it is written (Psalms 120:3-4): 'What will He give you and what will He add to you, you tongue of deceit? Sharpened arrows of the Mighty One, together with coals of the broom bush.' 'the Mighty One' is none other than the Holy One Blessed be He, as it is written 'the G-d who is great and mighty, etc.'" (We find the same in Arachin 15b.) And how much must the man of heart strengthen himself and stand guard against this bitter sin, in reflecting upon this awesome punishment, which the great prison house of Gehinnom itself is powerless to inflict unless it also be assisted from above!

The third aspect concerning which man's conduct on earth will be examined is alluded to by the dual spelling of 25,30 in connection with the discussion of the applicability of the law on redemption of houses sold permanently. The Talmud Erchin 15 discusses the advantage enjoyed by a creature who has been given the power of speech. Basing themselves on Psalms 120,3 "What can you profit, what can you gain, O deceitful tongue," our sages portray G–d as saying to the tongue: "All the other limbs of man are upright; but you are in a prone position. All the other limbs of man are an appendix to his body, but you are within his body. Not only that but I have surrounded you with two חומות, walls, one of bone (the teeth) and one of flesh." The secret dimension of the letter מ is that it appears both as an open letter and as a closed letter (the final ם which in its form is like a closed wall). When a person is circumspect about the use of his tongue, the mouth acts like a dosed wall. When he is careless, his mouth is like a wall which has broken down. This is alluded to in the final, closed ם in the middle of the word in Isaiah 9,6: למרבה הפשרה ולשלום אין קץ, "In token of abundant authority and of peace without limit." In the verse just quoted, our Rabbis perceive that it is human nature to look for faults in others, i.e. "a broken wall" while considering themselves like a "closed wall," i.e. free from faults. This is the deeper meaning of what our sages (Baba Batra 2a) describe as היזק ראיה, damage caused to a person by invading his privacy by looking into his domain [a kind of peeping. Ed]. It is the moral background to the Mishnah in the above mentioned section of the Talmud in which the contribution a neighbor has to make to a wall dividing common property is discussed. The purpose of the wall is to prevent such damage. In other words, the so-called damage is due to the tendency of people to discover something discreditable about their neighbors. We see that even halachically the wall is perceived as something that prevents damage of an abstract nature such as היזק ראיה.