CS Lewis in his book “Christian Reflections” (published in 1967), wrote, "The dominant impression I get from reading the Psalms is one of antiquity... In that momentary proximity they are almost shockingly alien; creatures of unrestrained emotion, wallowing in self-pity, sobbing, cursing, screaming in exultation (p114) ... we shall find in the Psalms expressions of a cruelty more vindictive and a self-righteousness more complete that anything in the [Greek] classics. If we ignore such passages and read only a few selected favourite Psalms, we miss the point. For the point is precisely this: that these same fanatic and homicidal Hebrews, and not the more enlightened peoples, again and again -- for brief moments -- reach a Christian level of spirituality. It is not that they are better or worse than the Pagans, but they are both better and worse.” (p116, "Christian Reflections")

Lewis is right about one thing. The Psalmist’s claim that he is perfectly righteous is “shockingly alien” to us. We’re sinners, but here the Psalmist is saying that he’s without sin? Who can the Psalmist possibly be? Who can say the following?

I will behave myself in a perfect way ... I will not know evil. Psa 101:2,4

O Jehovah, judge me according to my righteousness, and according to my integrity in me. Psa 7:8.

And Jehovah has returned to me according to my righteousness, according to the cleanness of my hands before His eyes. Psa 18:24

Preserve my soul for I am holy. Psa 86:2

My comment: Obviously either David was under a delusion or this is Christ speaking. The New Testament interpretation is that Christ is the Psalmist. Compare Hebrews 7:26 with Psalm 26...

Hebrews 7:26
For such a High Priest was fitting for us: holy, harmless, undefiled, and separated from sinners, and having become higher than the heavens;

Psalm 26:6,9,11
I will wash my hands in innocency; so I will go around Your altar, O Jehovah, Do not gather up my soul with sinners, nor my life ...