Musar su Proverbi 18:8

And the sixth section: The complainer - King Solomon, peace be upon him, said (Proverbs 18:8), “The words of the complainer are like mitlahamim; and they enter the innards of one’s belly.” Its explanation is that a complainer is a man whose way and whose nature is to always complain, get angry and find (movements) [pretexts] about this fellow, regarding his actions and his words - even though his fellow is innocent towards him, and did him no harm in anything. And he judges everything unfavorably, and not favorably; and anything inadvertent he makes volitional. And he surely makes himself like the oppressed and beaten, and as if the sin of his fellow is heavy upon him; whereas he is the hitter and the beater, as his words “enter the innards of one’s belly.” For the one who places complaints in front of his fellow when he did not touch him and only did good to him brings a storm to the heart. And behold he is like one who throws darts that “enter the innards of one’s belly.” The word mitlahamim is [spelled] inverted, [and is as if it were] mithalmim (they are beating); [and like the inversion of] simlah [and] salmah. And it is like the usage (in Proverbs 23:35), “they beat me (halamuni), but I was unaware.” And it is as if he was saying, “It is like the words of the complainer, they are beating me.” And his saying, “They are beating,” in the plural, is because the complainer includes [all] complainers - like (in Jeremiah 11:15), “the sacral flesh will pass away from you” (which is in singular, but refers to the many); [and] (Isaiah 45:8), “triumph sprout.”
King Solomon, peace be upon him, also said (Proverbs 16:28), “and a complainer separates his friend.” He means to say that he separates his friend and his companion from himself, as they cannot endure his friendship. And our Rabbis said (Derekh Eretz Zuta 9), “Do not proliferate complaints, so that you will not come to sin.” And many times, the complainer will be ungrateful for the good and will even consider it bad, so he will return the good with bad. And it is stated (Proverbs 17:13), “He who repays good with evil will not have evil leave his home.” And sometimes he will think about God’s kindnesses, that they are for vengeance and retribution - like that matter that is stated, (Deuteronomy 1:27), “You sulked in your tents and said, ‘It is because the Lord hates us that He brought us out, etc.’” Hence, distance yourself from the path of the complainers, for they have twisted their paths - anyone who walks in it will not know peace. Rather teach your tongue to judge favorably, and justice will be the girdle of your loins.