I had an exchange with "Jaywill" on Spirituality some years ago on this topic and was advised that the human race had to be destroyed in The Flood
because, apart from Noah and his immediate family, the entire human race had become inter-bred with foul demons / fallen angels known as Nephilim. So really it was not people that died in The Flood
but half demons and The Flood
saved humanity for another shot at salvation.
Why did God send the judgment of the Flood in the days of Noah? The strange events recorded in Genesis 6 were understood by the ancient rabbinical sources, as well as the Septuagint translators, as referring to fallen angels procreating weird hybrid offspring with human women-known as the Nephilim. This was far more than simply a historical issue, the unique events leading to the Flood are a prerequisite to understanding the prophetic implications of our Lord's predictions regarding His Second Coming. This is believed to have happened both before the Flood and after the flood. In Israel at the time of David, and before this when the Hebrews were scouting the land - there were tribes of these Nephilim - Goliath the giant of Gath is one example. Early Church fathers also understood this to be the case. It appears that these fallen angels were active all over the world. These bizarre giants living on the earth are also echoed in the legends and myths of nearly every ancient culture. The ancient Greeks, the Egyptians, the Hindus, the South Sea Islanders, the American Indians, and most all the others have these legends.
The Hebrew word Nephelim (plural of nephel) (Nephilim) literally means "rejects". The Hebrew plural for Nephites would be "Nephi'im".
Satan probably put this plan to interrupt Abraham's "seed into motion, as soon as it was made known that the Seed of the Woman (Christ) was to come through ABRAHAM.
Genesis 6:4 (Original KJV):
There were giants in the earth in those days; and also after that, when the sons of God came in unto the daughters of men, and they bare children to them, the same became mighty men which were of old, men of renown.
Bible Probe Note: Note the words; "and also after that". This probably tells of two "irruptions" by Satan's seed. One before Noah's time and another after the flood. They were called “The Watchers” in The Book of Enoch.
Their descendents, called Nephilim (translated "giants" ), were monsters of iniquity; and being superhuman in size and in their wicked character, had to be destroyed. Was this the one and only object of the Flood?
Genesis 6:12 (Original KJV):
And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth.
Only Noah and his family had preserved their pedigree pure from Adam (Genesis 6:9). All the rest had become "corrupt" (shachath). The only remedy was to destroy it defacto (Latin for "in fact" ), as it had become defacto destroyed.
This irruption of fallen angels before Noah's day was probably Satan's first attempt to prevent the coming of the "Seed of the woman" foretold in Genesis 3:15. If this could be accomplished, God's Word would have failed, and Satan's own doom would be averted.
Genesis 3:15 (Original KJV):
And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel.
also see Isaiah 7:14
[One has to appreciate that little bit of Latin translation - defacto ! ] http://www.bibleprobe.com/nephilim.htm
Obviously this is madness cubed but I was assured that Christianity and fidelity to the Bible demands acceptance of such material. More realistically, I would suggest that Christianity was never a true monotheist religion at all, but rather one in which the Christian God was perceived as more powerful than the others, who were redefined either as demons or angels, while humans instead of becoming gods could now become saints or evil spirits and influence us from beyond the grave for better or worse.