Of Ps 51:5,
...
5–8. He has inherited a sinful nature; and yet, so he is confident, God can and will make it conform to His desire. The emphatic ‘Behold!’ marks the beginning of a new stanza.
C H Spurgeon preached/wrote in Treasury of David on Ps 51:5,
EXPOSITION
Verse 5. Behold, I was shapen in iniquity. He is thunderstruck at the discovery of his inbred sin, and proceeds to set it forth. This was not intended to justify himself, but it rather meant to complete the confession. It is as if he said, not only have I sinned this once, but I am in my very nature a sinner. The fountain of my life is polluted as well as its streams. My birth tendencies are out of the square of equity; I naturally lean to forbidden things. Mine is a constitutional disease, rendering my very person obnoxious to thy wrath. And in sin did my mother conceive me. He goes back to the earliest moment of his being, not to traduce his mother, but to acknowledge the deep tap roots of his sin. It is a wicked wresting of Scripture to deny that original sin and natural depravity are here taught. Surely men who cavil at this doctrine have need to be taught of the Holy Spirit what be the first principles of the faith. David's mother was the Lord's handmaid, he was born in chaste wedlock, of a good father, and he was himself, "the man after God's own heart;" and yet his nature was as fallen as that of any other son of Adam, and there only needed the occasion for the manifesting of that sad fact. In our shaping we were put out of shape, and when we were conceived our nature conceived sin. Alas, for poor humanity! Those who will may cry it up, but he is most blessed who in his own soul has learned to lament his lost estate.
