John Gill

He shall reward evil unto mine enemies,.... That eyed him as Saul did; that observed his haunts, where he resorted, and who were with him, as the Ziphites did under Saul's direction; as the Word (m) here used for "enemies" signifies: the mischief they had devised for him, he believed, would be returned upon their own heads; the pit they digged they would fall into themselves; and the net they had spread for others their own feet would be taken in. This was true as of David's enemies, so of Christ's, the wicked Jews, who narrowly watched him to take every advantage against him; cut them off in thy truth; root and branch, as Saul, and his family, and his courtiers, quickly were, according to the truth of promises made to David, and of threatenings unto them. (m) "observatoribus meis", Junius & Tremellius, Piscator, Michaelis.