Albert Barnes Commentary Psalms 103:9

"He will not always chide; Neither will he keep [his anger] for ever." — Psalms 103:9 (ASV)

He will not always chide - Rebuke; contend; strive; for that is what the Hebrew word means. He will not always contend with people, or manifest his displeasure.

See the notes at Isaiah 57:16; notes at Psalm 78:38-39. This implies that he may chide or rebuke his people, but that this will not be forever. He will punish them; he will manifest his displeasure at their sins; he will show that he does not approve of their course, but he will show that he loves them, and does not seek their ruin.

Neither will he keep his anger forever - The words “his anger” are supplied by the translators, but not improperly. The meaning is the same as in the previous part of the sentence. He will not cherish hatred when the object of the chastisement is accomplished. It is not his character to retain anger for its own sake, or for any personal gratification.