the world was full of sin before Moses, so much that GOD destroyed THE world,'
GOD also told Cane that sin.
Gen 4:7 If thou doest well, shalt thou not be accepted? and if thou doest not well, sin lieth at the door. There is no sin without the law, not how could we say that they didn't have the law, they just didn't have it in stone, but they did have GOD's laws, They couldn't have a world full of sin without the law.