What is the meaning of "Ki Ad Tzedek Yashuv Mishpat"?
Rashi: Affliction (Mishpat) comes on Tzadikim until they are vindicated (get Kaparah) due to them.
Radak citing his father: There will come a time when Mishpat will return to Tzedek. It says Yashuv, for Mishpat was connected with Tzedek, and Resha'im separated them. Then Mishpat will return to its place and join with Tzedek.
Radak citing Ibn Ezra: In general, Mishpetei Hashem return to Tzedek, but there are exceptions. E.g. when rain descends, it is Tzedek and Chesed for the Klal who need water, but for a minority, it is harmful; this is not Tzedek.
Radak citing Yalkut Shimoni 850: This refers to Yitzchak; Mishpat returned to that Tzadik - "va'Tichhena Einav me'R'os" (Bereishis 27:1).
Malbim: This explains why Hashem delays punishment of a Rasha, for Mishpat (letter of the law) veers from its path to go to Tzedek. Tzedek considers the time and place - beyond the letter of the law. Sometimes absolute Mishpat obligates a Rasha to be punished, but it is delayed due to Tzedek - due to a Mitzvah he did, merit of his fathers, or Tzadikim that will be born to him, or perhaps he will repent. Or, it delays until his measure [of sin] is filled, or he is a Tzadik compared to his generation, who are bigger Resha'im. Mishpat will not punish him until Tzedek agrees.
Why does it say "v'Acharav Kol Yishrei Lev"?
Rashi: After the Mishpat (afflictions cleanse the Tzadikim), all the straight-hearted will gather, for they will receive their reward.
Radak citing his father: All with a straight heart will chase Tzedek. - Radak citing Ibn Ezra - they are truly Am Hashem.
Radak citing Yalkut Shimoni 850: The reward of Tzadikim is given after them. The reward of Resha'im is given before them - "u'Meshalem l'Son'av El Panav Leha'avido" (Devarim 7:10).
Malbim: Also all straight-hearted, when they want to receive their reward according to Mishpat, their path will veer to Tzedek. Sometimes, according to Mishpat it is proper that the Tzadik be rewarded immediately, according to Tzedek it is not, e.g. due to a previous sin, or to bequeath his merit to his children. The straight-hearted choose conduct based on Tzedek. Even though when the Rasha succeeds, it looks like Mishpat is crooked, afterwards Mishpat will return to Tzedek, and the straight-hearted will see Hashem's virtue in His Mishpatim.
