All for naught?

So, the lord flooded the earth killing everything because in Gen.6:5 “And God saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually.”  This I guess is a good reason but after the flood he says to Noah Gen.8:21 “And the LORD smelled a sweet savor; and the LORD said in his heart, I will not again curse the ground any more for man's sake; for the imagination of man's heart is evil from his youth; neither will I again smite any more every thing living, as I have done.”  It seems god murdered everything because man was evil and then decided men were still evil so he will not kill everything again.  Couldn’t he see this coming?  I think it is a little late to come to this revelation.  Another case proving that god is not omniscient.