Proverbs 20:28 - Joseph Benson’s Commentary on the Old and New Testaments

Mercy Clemency to offenders, and bounty to worthy indigent persons; and truth Faithfulness in keeping his word and promises inviolably; preserve the king Because they engage God to guard him, and gain him the reverence and affections of his people, which is, under God, a king's greatest safety and happiness. And his throne is upheld by mercy Which is again mentioned, to show that although to exercise mercy be an act of grace, and therefore, in some sort, free, yet princes are obliged to it both by their duty and by their interest, because it is a singular means of their preservation.