Which Sarim chased him?
Radak: Officers of Yisrael with Sha'ul or Avshalom.
Why does it say "umi'Devarecha Pachad Libi"?
Radak #1: I fear Your words, not to transgress one of Your Mitzvos, more than I fear them (Sarim who chase me).
Radak #2: They chased me and thought to remove me from Your Torah - "Lech Avod Elohim Acherim" (Shmuel I, 26:19). Even so, I feared to transgress Your Torah.
Radak #3: If this refers to Avshalom's officers, my heart feared lest I fall in their hands due to Your word "Hineni Mekim Alecha Ra'ah mi'Beisecha" (Shmuel II, 12:11).
Malbim: (a) I do not fear those who pursue me 1 , since my heart fears Your word. This great fear removes from my heart smaller fear, just like a man does not fear a bee when a lion is in front of him. The Chacham (Chovos ha'Levavos, Sha'ar Ahavas Hashem) said 'I am ashamed from fear of Hashem, that I should fear other than Him.' (b) Since I fear [the word of] Hashem, He will guard me from people, and they cannot harm me.
Malbim: They pursued me "Chinam" - it accomplished nothing.
