Why did he say "Zu Shaduni"?
Rashi: [Guard me] due to this, that my enemies plunder me. Radak - "Zu" is like Asher. The same applies to "Zu Chatanu Lo" (Yeshayah 42:24).
Malbim: David has two kinds of enemies. (a) Some wanted to make him sin, and cast him away from Yir'as Hashem. He asked Hashem to guard him like Ishon Bas Ayin from "Resha'im Zu Shaduni" - they plunder this (the black of my eye), for they want to blind me, i.e. tilt me from the path of light to darkness. Just like Hashem put much guarding on the eye, for it is small, clear, full of wonders and illuminates the entire body, so guard my clear Nefesh, Bas Elokim, from stumbling in sin. Also refer to 17:9:2:2.
What is the meaning of "Oyvai b'Nefesh Yakifu Alai"?
Rashi: [They surround me] to take my Nefesh (life).
Malbim: [Hide me in the shade of Your wings, from] the second kind of enemy, who wants to kill me.
