Here is the account as written in genesis.

Gen 6:5-13

5 The Lord saw how great man’s wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time. 6 The Lord was grieved that he had made man on the earth, and his heart was filled with pain. 7 So the Lord said, “I will wipe mankind, whom I have created, from the face of the earth — men and animals, and creatures that move along the ground, and birds of the air — for I am grieved that I have made them.” 8 But Noah found favor in the eyes of the Lord.

9 This is the account of Noah.

Noah was a righteous man, blameless among the people of his time, and he walked with God. 10 Noah had three sons: Shem, Ham and Japheth.

11 Now the earth was corrupt in God’s sight and was full of violence. 12 God saw how corrupt the earth had become, for all the people on earth had corrupted their ways. 13 So God said to Noah, “I am going to put an end to all people, for the earth is filled with violence because of them. I am surely going to destroy both them and the earth.

NIV

The Bible tells us that God flooded the world because mankind had become completely corrupt.  Casting Adam and Eve from the Garden reinforced the seperation from God that their sin had begun.  I believe that since they and their children had no moral bckground or compass and no continuing relationship with God, all of mankind descended into total depravity and not only could they not do good but they could not tell good from evil.

That probably seems exaggerated.  How could anybody do nothing but evil?  Wouldn’t someone do something good sometimes?  The answer is, without God we are not capable of doing good.  Even things that seem good, to a person without God, springs from selfish impulses and therefore are rooted in evil motives. 

Does that mean that everything that I do because I want to is evil?  It is likely but not necessarily.  Notice that all of the foregoing is based on what human beings are without God.  With God, we are increasingly capable of doing good.  Our motives become increasingly Godly.  The work of the Holy Spirit is to gradually mould and shape us into Godly creatures who love God.  With that love for God, we become more Godly.  As we become more Godly, the very impulses of our hearts become more pure and our behavior becomes more Godly.  Without God who we are becomes more brutal and carnal until we reach the total depravity of the world in the days of Noah.

I believe that God believed that Adam and Eve and their progeny would be so heart-wounded at being kicked out of the Garden of Eden, that they would repent and try to remember who they were when they walked with God.  But in the end, selfishness won the day and evil became the standard for all mankind.

So the question is, what will happen if the world sinks so far into evil once again?  God promised not to ever flood the world again.  What might He do next time?