Why is the whole world now guilty before God?
The whole world is now guilty before God because the Law was given, making sin unmistakable. Before the Law, people were not held accountable to a written standard, but once the Law came, there was no excuse for adultery, thieving, coveting, and other transgressions. Thus all the world became responsible and stands guilty.
Les explains that before the Law was given, people could not be judged for breaking a written Law, since no Law existed. As he puts it, you cannot break a Law if there is not one. That is why those before the Flood were not held to the same accounting. But as soon as the Law was given, the whole world became responsible. Now there is no question: adultery is sin, thieving is sin, coveting is sin, and so on through the Ten Commandments. The excuse 'I didn't know there was a Law against that' no longer works. The Law was added because of transgressions until the seed should come, and it made everyone guilty before God.