While the evil of man might explain why God destroyed man in the Flood (Gen 6:7), why allow him to continue on after that, and why bother with the destruction, if he is going to continue to be wicked after the Flood (Gen 8:21)?

In other words, after the Flood, God’s assessment of the merits of humanity seems little changed: “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” (Gen 8:21). If he is going to preserve man whose “heart is evil from his youth,” why destroy him in the first place? Perhaps part of the answer is that man would, ever after that, never be quite as corrupt as the antediluvians were. In other words, while it was very true that man was a fallen sinner “from his youth,” it does not follow from that sad description that “every imagination of the thoughts of his heart was only evil continually” (6:5); the three words I italicized here represent a significant difference. With the exception of Noah (and perhaps his family), man was utterly without merit. This suggests that the Flood was intended for, and successfully served a moral purpose, namely, it improved mankind’s moral stature. Man was never so completely corrupted as before the Flood. Even in the end times, there will always be a faithful remnant beyond just eight people.