Did people before the law have a sin nature?
Yes. From Adam to Moses, before the law, sin or Adamic natures was in the world. The pre-flood people had the old sin nature, and those between the flood and the giving of the law likewise had a sin nature; they were no different than we are.
Les teaches that verse 13 is a parenthesis describing the period until the law, from Adam to Moses, about 2,500 years. During that time sin, or Adamic natures, was in the world. The pre-flood people had the old sin nature, and the people between the flood and the giving of the law had a sin nature too, no different than we are. Sin is not imputed when there is no law, yet the sin nature was still present.
(For until the law sin was in the world: but sin is not imputed when there is no law.
More Questions
Why did God not impute sin before the law was given?
Before the law was given, God did not impute people's acts of disobedience because they were not per se breaking a law, even though they went against conscience.