Three ways of seeing the end of mankind

Genesis 6:4-5

4 • The Nephilim were on the earth in those days,
and also afterward,
when the sons of God came in to the daughters of man
and they bore children to them.
• These were the mighty men who were of old,
the men of renown.
5 • The Lord saw
that the wickedness of man was great in the earth,
and that every intention of the thoughts of his heart
was only evil continually.

The 'afterward' was when giants were found in Canaan by the spies sent out in preparation for entering the land to conquer it — ([Numbers 13:33] "And there we saw the Nephilim (the sons of Anak, who come from the Nephilim), and we seemed to ourselves like grasshoppers, and so we seemed to them.") — so when Moses wrote after the debacle caused by rejecting the minority report he was anticipating the sort of overwhelming defeat for the 'afterwards' giants as was wrought on their predecessors by the universal flood. Before the flood, mankind in general had not seen things that way at all, of course, and these Nephilim were renowned for all their outward glories. From the outside it must have appeared that God's threat of death (and his promise of salvation) was being nullified by the inextricable mingling of the seed of the woman with the seed of the serpent ( c.f. Genesis 3:15) but God, looking into the mind of man, saw neither great force nor great worth. He saw nothing good whatsoever.