James 4:17 in context does not teach what you desire. James is talking about some who go about boasting what they will accomplish the next day. He clearly states that such boasting is evil and that the promise of tomorrow is not theirs to determine, only God's. It is in THAT context that "to him that knows to do good..." is stated. It should NOT be disseminated into what you would carry it.
Romans 5:13 in context would disagree with your conclusion.
Just because the law was not written down in stone that someone could point to and cite the wrong didn't mean it was not imprinted upon the people living before Moses.
Because the law was imprinted, all people were held accountable - they died (wages of sin is death). In fact, only 8 were saved and that by the direct intervention of God.
It is an opinion only that the thief (I am assuming you mean the one who ask the Lord to remember him and not the one who jeered) had not sinned. The focus of that section is upon Christ. There is no statement as to what the thieves said when their legs were broken.
I totally agree that a person should work to sin less each day. It is true that some can "work out their salvation" with seemingly less problems in this carnal world system.
However, as John stated, "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."
A problem comes with the thinking that FAL sets out in recognizing the condition of the believer before God, and the condition of the believer in this world system.
John never says that a person can live a perfect life without sin in this world, but he does state that a person can live a perfect life without sin in this world.
Does that sound like double talk?
Look through a window. What do you see? Now stand were you saw and look through the same window. What do you see? Did you see the window or look through the window? Perhaps you did both.
From God's perspective (God looking at man through the window of Christ), man is sinless, perfected in Christ, and not the least bit held accountable for or to the standards of the law.
"Whosoever is born of God doth not commit sin; for his seed (Christ - the Word) remains in him: and he cannot sin, because he is born of God."
From the human perspective (man looking at God through the window of Christ), man is ashamed that he is entrusted as an ambassador for Christ in such a faulty, frail, and sin prone condition.
"My little children, these things write I unto you, that ye sin not. And if any man sin, we have an advocate with the Father, Jesus Christ the righteous: And he is the propitiation for our sins: and not for ours only, but also for the sins of the whole world."