Why did he say that Hashem is Tzadik?
Radak: Just like You are Tzadik, also Your Mishpatim are Tzedek.
Malbim: All His Mishpatim are Tzedek based on Din - paying reward to the Tzadik and punishing the Rasha.
Why does it say "v'Yashar Mishpatecha"?
Malbim: Even though the Rasha is paid promptly for his merits, and the Tzadik's reward is delayed, this is proper, to delay the Tzadik's reward until the world to come, which is eternal, and to pay the Rasha in this world to make him lose the world to come (refer to 73:18:1:3 and the note there).
Why does it say "Yashar" (singular)?
Radak #1: Each of Your Mishpatim is straight.
Radak #2: This refers to the Klal of Your Mishpatim.
