I am just going to cover one aspect about the flood that most poorly taught people never begin to understand. The "sons of God" who had sex with the daughters of men? These were angels that left their positions of authority and succumbed to lusts they did not wish to deny.
"Now it came about, when men began to multiply on the face
of the land, and daughters were born to them, that the sons
of God saw that the daughters of men were beautiful; and
they took/'seized passionately' to them women of all whom
they examined and selected." Gen 6:1
Sons of God only pertained to angels in the OT. We do not see believers being called "sons of God" until the church age began because we now have the indwelling Holy Spirit to make us so. . And, the Hebrew does not say they "married them" (as some translators present it to us.) It was an intense and passionate in action. No marriage is mentioned in the Hebrew.
Why did God need to kill off all the human race except Noah and his family? The angels had infiltrated the small human race (as it existed at that time in its infancy) that to allow that hybrid man to continue the promise of the Messiah (who had to be true humanity in body) could not have been brought into the world. Only Noah and his family remained free from the tainted DNA because they listened to God's preaching through Noah.
If God did not destroy the hybrid human race that was taking place just before the Flood? Jesus in pure humanity could not have been born as a man. Therefore, God had to wipe out that hybridization of men for that reason... besides all the terrible evils they were doing because of their half angelic - half man - bodies.