Why did David compose this Mizmor?
Malbim: David says that he is not a hypocrite full of evil inside, and externally a flatterer. Such people do like Zimri's act, and request reward like Pinchas! David's heart is faithful to his G-d, and there is no evil in his deeds. He is clean in action and thought. Therefore, he requests that what happens to flatterers should not happen to him, both regarding spiritual happiness and success in eternal life.
What judgment does David request?
Radak: See whether my heart is straight with You; judge me according to my heart.
Malbim: Make known my Mishpat. You know that I am a Tzadik, both in deed - "b'Sumi Halachti" - and in my mind and Emunah of the heart - "uva'Hashem Batachti", a strong trust that I will never falter.
Here David asks Hashem to judge him. Elsewhere (143:2), he says "v'Al Tavo v'Mishpat"!
Rashi (from Shocher Tov): Judge me when You judge Resha'im, for compared to them I am a Tzadik. Do not judge me when You judge Tzadikim.
Radak (2): Judge me according to my heart - it is always straight. Do not judge me based on my deeds; sometimes they are not good.
Why did he say "b'Sumi Halachti uva'Shem Batachti"?
Radak: I went in the purity of my heart, and in all that I had, I trusted in Hashem. Therefore, it is not proper that I falter.
Malbim: Refer to 26:1:2:2.
