What is the meaning of "b'Hishafto Yetzei Rasha"?
Rashi: When [my hater] is judged in front of You, his verdict should be liable and [he is called] a Rasha.
Radak: If he (the head of the Resha'im who hate me) has a judgment with anyone, he should come out a Rasha. I.e. he should stumble in his words and claims.
Malbim: Sha'ul will judge me evilly, and they will prosecute against me, and sentence me to death.
Why does it say "u'Sefilaso Tihyeh la'Chata'ah"?
Radak #1: When he prays for himself, his prayer should not be a merit, rather, like a sin and a liability.
Radak #2: "La'Chata'ah" is like "El ha'Sha'arah v'Lo Yachati" (Shoftim 20:16) - it should not be accepted.
Malbim: The custom was to supplicate in front of the judge for Chesed. They say that also [David's] supplication should be like sin, and not be accepted.
