In BT Bava Basra 10b we read that Rabban Yochanan be Zakkai asked his students what this verse means. Why is kindness called a sin? What is the reason for the contrast between a singular nation and plural nations or peoples?
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Rabbi Eliezer answered that the singular nation is Israel alone. And while kindness may sometimes be shown by gentiles, when they discover that they do not receive the reward for it that motivated them they may regret the act.
It becomes clear that King Solomon is saying that, without the redeeming word of G-d, given as covenant to Israel alone, human beings are still under the sentence of death and even should they do acts of kindness all their acts are still judged as sin, for they are acts done by enemies of their Creator and by rebels against the true G-d.