Proverbs 24:17,18 - Joseph Benson’s Commentary on the Old and New Testaments

Rejoice not when thine enemy falleth Namely, into mischief or trouble, as in the former verse; please not thyself in his destruction. This plainly shows that the love of our enemies is a precept of the Old Testament, as well as of the New: see Exodus 23:4-5. Lest the Lord see it, &c. “For though nobody sees it, God does; and such affections are so displeasing to him, that they may provoke him to translate the calamity from thy enemy unto thee, and thereby damp thy sinful joy with a double sorrow.”