Charles Ellicott Commentary Proverbs 10:18

"He that hideth hatred is of lying lips; And he that uttereth a slander is a fool." — Proverbs 10:18 (ASV)

He that hideth hatred ...—This would be more correctly translated, He that hides hatred is a mouth of falsehood: he that spreads slander is a fool (khesîl:Proverbs 1:22). (For the construction, he ... is a mouth of falsehood, compare to note on Proverbs 8:30; and for the sentiment, David’s complaint, Psalms 41:6).

Is a fool. —For he does mischief to his neighbour, and only gets ill-will for himself.