Noah's flood was sent because people were breaking laws. The point is that sin, in any of its manifestations, brings death. Sins of omission kill as well as sins of commission. Violation of one law of God kills, as well as violation of another law. The distinction between theirs and Adam's would be non-existent if they had died because of Adam's sin.
People die because of their own sins.
Adam was the father of the race as it fell under bondage to sin. However, the superiority doesn't exist in the number saved as opposed to the number lost. Grace has more than enough power and adequacy to offset all that sin is able to do. No matter how sin triumphed over men, grace had the antidote for it. It just takes one sin to condemn, and yet the benefits of Christ's death can justify one of many sins.