Just Misunderstanding
Compare Genesis 9:21 to Genesis 6:5
Then the LORD saw that the wickedness of man was great in the earth, and that every intent of the thoughts of his heart was only evil continually
It seems as if the rationale given by God for not destroying man (in Genesis 8) is the very reason He gave for mans destruction in Genesis 9. Listen closely and we can almost hear the Bible conspirators chanting in slapdash harmonies, “Contradiction! Contradiction!” Let the suspicious and the disbelieving know that these two verses are not contrary to one another. God is not fickle, He cannot oppose Himself, nor does He ever change His mind.
A Protocol is Established
For our benefit, God communicates His knowledge of mans perpetual sinful nature, the likes of which could not be destroyed by flood waters alone–God had only obliterated that which was beyond restoration. Sin we recall, is the singular thing that separates us from God and He will go on to say of our blood in Genesis 9:5, “I will require.” We now appreciate that without the shedding of blood there is no remission for our sin and take notice of what blessed the Lord’s heart:
And the LORD smelled a soothing aroma. Then the LORD said in His heart, “I will never again curse the ground for man’s sake…”
Noah sacrificed and God responded. The sacrifice was pleasing to God and for the first time perhaps we see that what took place in chapter 8 did not occur in chapter 6. Therefore, the difference between the two events is that total surrender in the form of sacrifice has been factored into the equation, setting the table for the ultimate in remission of sins through the blood of Jesus Christ.
By him therefore let us offer the sacrifice of praise to God continually, that is, the fruit of our lips giving thanks to his name. Hebrews 13:15
He Dwells -- The B'log in My Eye 