What does David discuss in this Mizmor?
Radak: He discusses the protocol of his kingship, how He conducts with Yisrael. It tells also what he does in his free time, when litigants are not in front of him and he isolates himself from people. Then he engages in Chachmah, and turns his heart from worldly matters and prepares it to receive Ru'ach ha'Kodesh.
Malbim: He discusses his conduct both in matters between him and his G-d, and between him and people.
Why does it say "Chesed u'Mishpat Ashirah"?
Rashi (from Shocher Tov): When You do Chesed with me, I praise You 'Baruch ha'Tov veha'Metiv.' When You do Mishpat with me, I praise You 'Baruch Dayan ha'Emes.' Either way, I sing to You. Radak - "Hashem Nasan va'Shem Lakach" - whether He gave or took, "Yehi Shem Hashem Mevorach" (Iyov 1:21).
Radak: I sing to You for the Chesed and Mishpat that I do to Yisrael. You made me king, and gave to me a good heart to rule over them with Mishpat and Tzedakah.
Radak citing Shocher Tov: Chesed and Mishpat are with You - "va'Yigbah Hashem Tzevakos ba'Mishpat veha'Kel ha'Kadosh Nikdash bi'Tzdakah" (Yeshayah 5:16).
Malbim: Hashem's conduct with him, whether Chesed or Mishpat, has no bad in it. The good that He does is amidst Chesed, and not due to obligation - "Mi Hikdimani va'Ashalem 1 " (Iyov 41:3). The evil is due to Mishpat, due to bad deeds, to atone for them. Therefore, I sing for both of them. Also the Mishpat is due to His Hashgachah and Tzedek; He oversees me to eradicate my Tum'ah from me.
Pesikta Rabasi (21): Who circumcised his son before I gave to him a son? Who fulfilled Mezuzah and Ma'akah before I gave to him a house? Who said Shirah to Me before I did miracles for him?
