Why would God create a “very good” world only to allow it to fall so far that he had to destroy it (Gen 6:7)? Does not the talk of God’s “repentance” imply he had no knowledge or control over the state of the world?

First, let us get clear on this: to say “it repented the Lord” (i.e., “the Lord repented”) does not mean either that God admitted to himself that he had done something wrong or had made a mistake, or even that he had changed his mind on some point of principle. What had changed was man’s moral merits, and God’s attitude toward man reflected this moral fact. This still left a problem, which is really a variant on the problem of evil, unsolved. There is little in the Bible, as far as I can tell, that clearly reveals God’s purposes in permitting evil, although it is absolutely clear that he does permit it and also that he even achieves important purposes by permitting it. The book of Job clearly implies we must not expect to learn God’s purposes. But a theory that is at least consistent with the Bible makes it at least plausible: God did the least he could to help rebellious man until repentant man could be redeemed and purified by the perfect sacrifice. God kept a righteous remnant alive so this was possible—and thus he did not repent of his original purpose in creating man, namely, to create intelligent beings, acceptable to the Lord, who would freely glorify and take delight in the Lord. There is much more to the theory than that, but it is enough to suggest the answer to this question. By the way, the word translated “repent,” נָחַם or nacham, is the same as that used by Lamech in his prophesy saying Noah will “repent (nacham) us”; it is as if God were saying, “You were right, Lamech: Noah would bring “comfort (nacham)” in the sense of “repentance (also nacham),” although not in the way you meant. My repentance would put an end to your work and toil—with death.”

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.

What therefore was God’s purpose in (nearly) destroying man (Gen 6:7)?

I think it is fair to say that he was, literally, putting the fear of God into mankind ever after. His act was that of a sovereign exercising absolute prerogative over his own creation. He was, once and for all, establishing the absolute sovereignty of the Lord. This is a thing that God does over and over in the Bible: he demonstrates his sovereignty over one aspect of the creation, and that, typically, fairly early on in the narrative. Other examples include exile of Adam and Eve from the Garden, and of Cain from his presence; of the Tower of Babel, an attempt to raise man to the level of “the gods”; of the thoroughly corrupt but prosperous cities, Sodom and Gomorrah, of the (then) green rift valley east of the Promised Land; of the Egyptians, the first great empire of the Bible; of those who would use pagan idols to worship God, beginning with Rachel; of Nadab and Abihu, destroyed as a sign of his sovereignty over the priestly activities; of the wandering men who refused to trust in God’s guarantee of the Promised Land; later, of the fall of Israel and Judah, showing his sovereignty over the Promised Land when the Hebrews failed to keep up their part of the covenant; ultimately, he will demonstrate once and for all his total sovereignty over all creation at the end of time. So in the case of Noah, he chose to (all but) destroy man in order to clarify to wicked man from nearly the beginning, his sovereignty over his creation: we should not expect to be able to live in such wickedness forever. In the Psalms in two places there are two instances where the image of a flood, or of The Flood, is used to illustrate God’s sovereignty. “The Lord sitteth upon the flood; yea, the Lord sitteth King for ever.” (Ps. 29:10) Little could be clearer than that. But consider also: “Your throne is established from of old; You are from everlasting. The floods have lifted up, O Lord, the floods have lifted up their voice; the floods lift up their waves. The Lord on high is mightier than the noise of many waters, yea, than the mighty waves of the sea.” (Ps 93:3-4)

“But wait,” a nonbeliever will insist. “It seems unnecessary for an entity so powerful to have to demonstrate his sovereignty by total destruction (Gen 6:7). Why would he want to?”

Is it due to a kind of divine sadism? Or does it demonstrate some sort of emotional pettiness and neediness? There is a sound question lying behind the objectionable wording, but I wanted to put the question in this sort of objectionable way because it does capture a nonbeliever’s natural reaction to the Flood story that it is important to address. So let us be very clear: the point of God demonstrating his sovereignty is neither (a) because it is sadistic fun for him to hurt us, nor (b) because he had some sort of perverse (really, it would be quite evil) need to assert his mastery over us. No, in each case in which God demonstrates his sovereignty, the demonstration is for the benefit of all humanity to come, as recorded in the Bible. The Flood disaster shows once and for all that God’s patience will not last forever, and that he reserves to himself the absolute moral right to, as it were, pull the plug on the whole game that he started. This is just one reason we ought to have the fear of God. That fear, as the Bible reiterates time and again, keeps us on the straight and narrow. It is also worth reiterating that the antediluvian people, like the later Sodomites and the Canaanites, were unusually wicked: “every imagination of the thoughts of his heart was only evil continually.” (Gen 6:5)

It seems obvious enough why God feels grief at man’s evil, and this is why he might wish to destroy man; but why “beast, and the creeping thing, and the fowls of the air” (Gen 6:7)?

There may be two reasons. The broader animal kingdom was created for man, who had responsibility for it; with man destroyed, it would make sense to destroy animals. A second reason, perhaps, is that these creatures too had been corrupted in the Fall, even as the land has sprouted “thorns and thistles.” A remark in Gen 6:11 is consistent with this: “all flesh”—not just human flesh—“had corrupted his way upon the earth.” That would certainly be true if the wolf originally lay down with the lamb, but became a carnivore after the Fall. This is suggested by the notion that nature would once again live in peace in the new creation: “The wolf also shall dwell with the lamb, and the leopard shall lie down with the kid; and the calf and the young lion and the fatling together; and a little child shall lead them.” (Isa 11:6)