The law could not destroy the sin nature all it could do was temporarily control or suppress the sin nature in them. It is quite obvious that the old sin-nature had to be controlled and that was why God gave the Mosaic law. This is the meaning of: "So then with the mind I myself serve the law of God; but with the flesh the law of sin," Romans 7:25. The law was given so that the soul mind could use the law to suppress or control the sin nature in them. The born-again believer thinks they do not attempt to control the new Christ in you nature but most unknowingly do. If the born-again believer attempts to live life by using the law or good morals etc. then this is nothing more than a method of the mind using morals to control The Christ in you nature, within. Christ will be automatically manifested when the believer who has the mind of Christ lives by the nature in them, Christ in you fulfills all morals.