Personally I think that God only wanted to destroy those individuals in the flood that knew everything about Him and then turned away from Him and His ways.
God created or lets say changed mans beastlike behavior and thinking into a more moral , aka. clean and spiritual one .. this is why I said that those who lived somewhere else that did not know Him , didn't have to die in the flood.
God created a new spiritual entity-land-kingdom for Himself and it didn't work out as He planed it ... so He washed it all away so to say and kind of started over with the few that He chose to be worthy to represent His kingdom.