Psalm 14:3 They are all gone aside, they are all together become filthy: There is none that doeth good, no, not one.
Romans 3 is simply quoting a truth from the Old Testament. All men have sinned. Notice a key phrase. They have all gone aside. In other words every man does what he wants. We turn to our own ways and not His. That is what sin is. It is me deciding, choosing, and thinking that I know what God knows.
God has a way and I am to submit to it. If I do my own thing then I am in sin!