What changed when the Law was given regarding the world's responsibility for sin?
When the Law was given, the whole world became responsible for sin, since there was now no question that adultery, thieving, coveting, and all the Ten Commandments were sin. Previously, without written Law, people could not be charged with breaking it; now all the world is guilty and without excuse.
Les teaches that before the Law, God could not put the wickedness of people before the Flood to their account because they were not breaking any written Law. They had no Law, and you cannot break a law if there isn't one. But as soon as the Law was given, the whole world became responsible because now there was no question: adultery, thieving, coveting, and all the way down through the Ten are sin. So all the world has now been made guilty because there is no excuse for adultery; they now know there is a Law against it. Les also points to Galatians 2:19, which says the Law was added because of transgressions until the seed should come, and it was ordained by angels in the hand of a mediator.