C. F. Keil and Franz Delitzsch on Proverbs 10:18

Pro 10:18 18 He that hideth hatred is a mouth of falsehood;      And he that spreadeth slander is a fool.

The lxx, καλύπτουσιν ἔχθραν χεῖλα δίκαια, which Ewald prefers, and which has given occasion to Hitzig to make a remarkable conjecture (“He who conceals hatred, close lips,” which no one understands without Hitzig’s comment. to this his conjecture). But (1) to hide hatred (cf. Pro 10:11, Pro 26:24) is something altogether different from to cover sin (Pro 10:12, Pro 17:9), or generally to keep anything secret with discretion (Pro 10:13); and (2) that δίκαια is a corrupt reading for ἄδικα (as Grabe supposes, and Symmachus translates) or δόλια (as Lagarde supposes, and indeed is found in Codd.). Michaelis well remarks: odium tectum est dolosi, manifesta sycophantia stultorum. Whoever conceals hateful feelings behind his words is שׂפתי־שׂקר, a mouth of falsehood (cf. the mouth of the fool, Pro 10:14); one does not need to supply אישׁ, but much rather has hence to conclude that a false man is simply so named, as is proved by Psa 120:3. There is a second moral judgment, 18b: he who spreadeth slander (וּמוצא, according to the Masoretic writing: he who divulges it, the correlate to הביא, to bring to, Gen 37:2) is a Thor fool, stupid, dull, כּסיל (not a Narr fool, godless person, אויל); for such slandering can generally bring no advantage; it injures the reputation of him to whom the דבּה, i.e., the secret report, the slander, refers; it sows discord, has incalculable consequences, and finally brings guilt on the tale-bearer himself.