Why is the very nature of mankind sinful?
The sinful nature of mankind is inherited from Adam. The whole sin problem began with Adam, and all humanity inherited it. People are not sinners merely because they break the law; rather, they break the law because they are sinners by nature. The law was given to reveal how sinful man is.
Les teaches that Paul, writing in Romans 5:20, says the law entered so that the offense might abound. Les explains this does not mean the law caused people to sin more, but that it caused them to understand how sinful they were. He illustrates that we are not sinners because we have broken the law; we break the law because we are sinners. The whole sin problem began with Adam and has been inherited, so the very nature of mankind is to be sinful. When the law came, all it could do was show man how sinful he really was. Les also calls the singular word "sin" the old Adam or the old sin nature, and notes that where sin abounded, grace did much more abound.