Self-Righteousness in the Psalms?

C.S. Lewis, in his “Reflections on the Psalms”, devotes some time to talking about what he thinks is blatant self-righteousness on the part of the Psalmists in many of our beloved Psalms.

For example, Lewis presents Psalm 7:3-5, 8 as an example of this ugly self-righteousness (and I must admit it is very hard to "excuse" the Psalmist's wording in verse 8 in particular):

“O LORD my God, if I have done this, if there is wrong in my hands, if I have repaid my friend with evil or plundered my enemy without cause, let the enemy pursue my soul and overtake it, and let him trample my life to the ground and lay my glory in the dust."

“The LORD judges the peoples; judge me, O LORD according to my righteousness and according to the integrity that is in me.” (Psalm 7:3-5, 8)

However, verses 1 and 17 of this chapter seem to show that this "self-righteousness" is ultimately grounded in God and His righteousness:

"Oh LORD my God in You I put my trust…I will praise the LORD according to His righteousness". (Psalm 7:1a, 17a)

Lewis thinks that we should learn from these instances of self-righteousness in the Psalms by trying to avoid doing the same in our own lives. Of course we want to avoid self-righteousness, however, I wonder if the Psalmists are really being self-righteousness at all. Are they not rather expressing as their own the righteousness that they have acquired of God (sanctification) by faith?

In Psalm 18 we see another example of this seeming self-righteousness, but below I have placed the questionable verses side by side with other verses within the context of the same chapter showing how the Psalmist ultimately depended upon God for everything:

Psalm 18:20-26a: 20 "The LORD dealt with me according to my righteousness; according to the cleanness of my hands he rewarded me." 21 "For I have kept the ways of the LORD, and have not wickedly departed from my God." 22 "For all his rules were before me, and his statutes I did not put away from me." 23 "I was blameless before him, and I kept myself from my guilt." 24 "So the LORD rewarded me according to my righteousness, according to the cleanness of my hands in his sight." 25-26a "With the merciful you show yourself merciful; with the blameless man you show yourself blameless; with the purified you show yourself pure"

Psalm 18:28-33, 35: 28 "For it is you who light my lamp; the LORD my God lightens my darkness." 29 "For by you I can run against a troop, and by my God I can leap over a wall." 30 "This God-his way is perfect; the word of the LORD proves true; he is a shield for all those who take refuge in him." 31 "For who is God, but the LORD? And who is a rock, except our God?-" 32 "the God who equipped me with strength and made my way blameless." 33 "He made my feet like the feet of a deer and set me secure on the heights." 35 "You have given me the shield of your salvation and your right hand supported me, and your gentleness made me great."

Take note, especially, of verse 31: "For who is God, but the LORD? And who is a rock, except our God?”. A self-righteous man deifies himself and tries to find his ultimate comfort and satisfaction in knowing that he is his own secure foundation. But this verse shows that the Psalmist recognizes no God “but the LORD” and no sure foundation “except our God”!

(All scriptures quoted from the ESV, emphasis mine.)