Malbim: Tzedek is in matters of Mishpat - the judgment is correct, and not crooked - "v'Shafat b'Tzedek Dalim" (Yeshayah 11:4). This is king's desire, for "Hen l'Tzedek Yimloch Melech" (ibid. 32:1).

R. Yonah: They despise only "doing evil" (12). One who refrains from evil in deed and speech, he is not despised. His Ratzon and love is for one who speaks straight. He chooses straightness and wants straightness and Emes to overpower in the land, therefore he speaks straight. He will not be silent to flatter, to delay [punishing] for breakage of Emes.

Malbim: Yesharim is a higher level than Tzedek. It is one who does straight beyond the letter of the law. It includes straightness of mindsets. The king loves this; it is more than desire.