David may have loved the law, but I wouldn't call him a law keeper...adultery...plotting murder..covering up his murder, numbering his troops against the advice of his general etc.
Paul didn't fare much better acknowledging himself as the chief of sinners.
Well, ... everybody
has sinned ...
"If we say that we have no sin, we deceive ourselves, and the Truth is not in us. If we confess our sins, He is faithful and just to forgive us our sins and to cleanse us from all unrighteousness. If we say that we have not sinned, we make Him a liar, and His Word is not in us."
(1 Jn. 1:8-10)
... and everybody
does sin.
"For we all
stumble in many things..."
(Jam. 3:2)
In order for it to be a
stumble, we must first be striving to walk upright (righteously).
We cannot live in habitual sin and expect Jesus to continue to wipe up our continual/intentional mess.
When we strive for righteousness, obedience to God's/Jesus' Commands, He sees our heart's intention and He supports our efforts with His Holy Spirit - guiding and giving strength to live holy lives; just as Jesus did.
"He who says he abides in Him (Jesus Christ) ought himself also to walk just as He walked."
(1 Jn. 2:6)
Jesus was 100% sinless when He took His last breath on the Cross of Calvary. None of us are, but we CAN strive to imitate our beloved Master. Every servant goes where his Master goes in the end - if he is a true servant.
All that is needed to decide is ... Who is your Master, the
World and the
approval of man ... or God and His Son who lived a life of obscurity that culminated in persecution and suffering.
"Therefore, since Christ suffered in the flesh, arm yourselves also with the same mind, for he who has suffered in the flesh has ceased from sin, that he no longer should live the rest of his time in the flesh for the lusts of men, but for the will of God."
(1 Pet. 1-2)
ETA: Paul referred to himself as the chief of sinners in reference to his time before he was converted on the road to Damascus. Before that, he was an outright murderer. I'd say he made up for it by penning more books of the Bible than anybody else.