Why is doing Mishpat Simchah for a Tzadik?
Rashi (citing Shocher Tov 3): It is Simchah for Hashem to do Din on Tzadikim, to make them merit the world to come. It is Simchah for Tzadikim that Hashem brings afflictions on them in order that they will merit the world to come.
R. Yonah: We are obligated to serve Hashem amidst Simchah - "Tachas Asher Lo Avadta Es Hashem Elokecha b'Simchah uv'Tuv Levav" (Devarim 28:47), "Ivdu Es Hashem b'Simchah" (Tehilim 100:2). A Tzadik rejoices when others serve Hashem, distance sin and do Mishpat. A Tzadik's Midah is to sigh if oppression of Aniyim, and improper Mishpat are seen in the land. He rejoices when the nation does Mishpat and pursues Tzedek and Emes. Simchah is arousal of the heart and spreading of Ru'ach on finding one's desire. It is greater than Gilah and Masos.
Sha'arei Teshuvah (4:12): Tzadikim pay their debt and sing to Hashem. A Mashal for this is an Aris (sharecropper) who owed the Ba'al ha'Bayis. The Ba'al ha'Bayis took the [entire] stack, and the Aris entered his house empty-handed, and rejoiced - the document is canceled. I paid my debt!
Malbim: Tzadik is one who conducts according to the laws of Tzedek. He knows that when Hashem does Mishpat, he will be vindicated.
How is it Mechitah for sinners?
Rashi: They do not put to their hearts to repent. The afflictions do not help them; they are pained without benefit.
R. Yonah: Mishpat scares sinners. When they see that in the city, they judge theft and extortion, they fear due to the theft in their hands. Aside from punishment for the sin in their hands, they will be punished for hating Mishpat and for their choices. In the city they do not rebuke and judge for every sin - "va'Ta'asu ha'Ra b'Einai uva'Asher Lo Chafatzi Bechartem" (Yeshayah 65:12). There is punishment for the choice, aside from for the act.
Malbim: Po'alei Aven are those who use physical strength to oppress and extort, the opposite of Tzedek. (Mishpat will break them.)
