1)
Why does it discuss a Tzadik?
1.
Radak: Now it explains the Mishpat, lest people err. They should not err about "Poked Avon Avos Al Banim", similarly "l'Son'ai"! A Tzadik is one who refrains from [transgressing] Mitzvos Lo Sa'aseh.
2.
Malbim: This refers to one whose nature is to be a Tzadik. Tzedek is a Kinyan in his Nefesh. He also does Mishpat and Tzedek in action. Mishpat is in Mitzvos Bein Adam l'Chavero, and Tzedek is in Mitzvos Bein Adam la'Makom.
2)
Why does it mention doing Mishpat and Tzedakah?
1.
Radak: He fulfilled Mitzvos Aseh.
