He was completely ignorant of God's Holy character. The table has completed flipped upside down. Thus God adds the law to cause man to begin the process of gaining knowledge about his sinful condition, God's Holy character and Satan's evil character. Not only did sin deceive man about himself, God and Satan, it now does so with respect to the law. For apart from the law sin was dead.
Sin is now alive and well under the law.
Before the law sin could not grow, it could not multiply itself for it had no opponent. However once the law was added, sin found its long sought treasure - a tool to use to increase its effects upon man. In fact the law gave strength, or power, to sin. With the law added, sin could and did abound. It was able to accomplish that for which Satan had desired when he deceived Eve in the garden.