The righteousness of God

Many believe that righteousness is about what one does or doesn’t do, as if what one does defines what , and who one is. Nothing could be further from the truth. In biblical terms righteousness means acceptable to God. King David in human terms was an adulterer and a murderer but Solomon said, Thou hast shewed unto thy servant David my father great mercy, according as he walked before thee in truth, and in righteousness. What does this mean? Why the need of mercy if “walked before thee in truth, and in righteousness. ” is true?