Malbim: The king's throne is prepared to do Tzedek - "Hen l'Tzedek Yimlach Melech" (Yeshayah 32:1). However, the throne needs officers and advisors, that via them Mishpat is executed. If [the king's] servants are Tzadikim, then the throne is like refined silver, free of dross. However, if there is an evil advisor among them, he will make Tzedek crooked, and the proper verdict will not emerge. Just like if a smelter wants to make a Kli from refined silver, he must remove the dross, so if they want to establish the throne in Tzedek, they must remove a Rasha who stands in front of him.