Why did he say "Elokim Shofet Tzadik"?
Rashi: You judge righteously.
Radak: This is like "Shafteni Hashem k'Tzidki" (verse 9); He judges the Tzadik according to his virtue, and the Rasha according to his evil.
Malbim: Even if externally, a sin is found in a Tzadik, Hashem judges also intent. It is unlike the same deed of a Rasha who angers Hashem!
Why is "Kel Zo'em b'Chol Yom"?
Rashi: This is when He sees the deeds of the Resha'im.
Radak: He judges the Tzadik, and judges one who angers Hashem every day, i.e. constantly, for he does not repent from his evil. How did Sha'ul anger Hashem? He disgraced Hashem's word and deed. He knew that David was anointed to be king after Sha'ul, and every day he chased to kill him, to nullify Hashem's word. Also, David was innocent; he did not sin against Sha'ul, and Sha'ul wanted to spill innocent blood. Also, Sha'ul swore "Chai Hashem" not to kill David (Shmuel I, 19:6, and now he sought to transgress this 1 !
We find that Hashem put a Ru'ach Ra on Sha'ul, and then he tried killing David (Shmuel I, 18:10-11, 19:9-10)! However, Sha'ul was not totally exempt due to Ones. The Gemara derives that his shame over killing the Kohanim atoned for him (Brachos 12b)! (PF)
