Why did David compose this Mizmor?
Malbim: It teaches that Hashgachah is drawn after clinging to Hashem. One who clings to Him, Hashgachah constantly clings to Him and guards him from all mishaps. Therefore, he does not fear evil - "Lo Ye'uneh la'Tzadik Kol Aven" (Mishlei 12:21), only when he ceases to cling, and turns from Avodas Hashem, like it says in Moreh ha'Nevuchim (3:53). Therefore, man should make one request, which includes all that he seeks - to constantly cling to Hashem, and nothing should cease this.
Why did he say "Hashem Ori v'Yish'i mi'Mi Ira"?
Radak: Since He is my light and salvation, I need not fear people! Affliction is like darkness, and salvation from it is like light.
Malbim: A person fears enemies (a) lest they attack him at night, when he does not see to beware of them. Therefore, it says "Hashem is my light." (b) Lest their army and strength is greater than his. Therefore, it says Hashem Yish'i - whom will I fear?!
What is "Ma'oz Chayai"?
Radak: It is a strong tower in which man takes refuge to save his soul, lest enemies kill him. Even one in a tower fears lest enemies conquer it, or besiege it, and he will starve. Hashem is Ma'oz Chayai, so I need not fear!
What is the difference between Yir'ah and Pachad?
Malbim: Yir'ah is from something known, e.g. open enemies. Pachad is from something unknown. E.g. perhaps haters covertly plot evil, to poison my food, unknown to me. "Hashem Ma'oz Chayai" - whom will I fear?!
