Human wives bore sons to their angel husbands, corrupting creation.
We have previously established that, in the Bible, the term "sons of God" refers to GOD's heavenly creatures, including angels and that certain angels left their assigned roles in order to marry beautiful human women.
1 Now it came to pass that men began to exist in great numbers on the earth, and daughters were born to them. 2 So when the sons of God saw the daughters of men were beautiful, they took wives for themselves of all they chose.
(Genesis 6:1-2 OSB)
The wives of these angels bore children to them.
These children were half-human, half-angel—a new species of creature that GOD had not intended nor created. For these impacted lineages, human DNA had been corrupted.
The Hebrew Bible refers to these hybrid people הַנְּפִלִ֞ים (Nephilim), which means "giants." (Note: While the term Nephilim appears later in the Hebrew Old Testament, these first giants were unique because they were direct descendants of angels.)
These half-angelic Nephilim grew into tall and mighty warriors who did not honor GOD, but were evil. Scripture indicates that they contributed significantly to the spread of wickedness on the earth.
Picking up where we left off in Genesis 6 above:
3 Then the Lord God said, “My Spirit shall not remain with these people forever, for they are flesh. So their days shall be one hundred and twenty years.” 4 Now there were giants on the earth in those days, and also afterward, when the sons of God came in to the daughters of men and they bore children to them. Those were the mighty men of old, men of renown.
(Genesis 6:3-4 OSB)
GOD was so grieved by the wickedness on the earth that He determined to destroy everything and start over.
5 Then the Lord God saw man's wickedness, that it was great in the earth, and every intent of the thoughts within his heart was only evil continually. 6 So God was grieved that He had made man on the earth, and He thought this over. 7 Then God said, “I will blot out man whom I created from the face of the earth, from man to cattle, and from the creeping things to the birds of heaven, for I am grieved I made them.”
11 Now the earth was corrupt before God and filled with unrighteousness. 12 Thus the Lord God looked upon the earth, and indeed it was corrupt; for all flesh corrupted their way on the earth.
(Genesis 6:5-7, 11-12 OSB)
The statements of Genesis 6:11-12 indicate that perhaps even the animals had been corrupted. All of this helps explain why GOD chose the flood to destroy everything.
The Bible doesn't give a lot of additional detail about these original Nephilim, but we do have an extended text from Ezekiel 31-32 that refers to them. That entire passage is too long to include here, so I'll summarize.
In Ezekiel 31, while pronouncing a coming judgment upon Egypt, GOD compared Pharaoh to a heavenly being named Assyria, who is described as the tallest tree whose glory reached to heaven. Because Assyria was prideful, GOD gave him over to destruction by others. The ending is curious:
13 All the birds of heaven rested on his fall, and all the wild animals of the field came upon his branches, 14 that no trees in the water may ever again exalt themselves in their greatness, nor establish their rule among the clouds, and that no trees which drink water may stand in their high position. For all were delivered to death, to the depths of the earth, among the sons of men who go down to the pit.’
15 “Thus says the Lord and Master: ‘In the day he went down to Hades, the deep abyss mourned him; and I stood over her rivers and restrained her abundant water. Lebanon mourned over him, and all the trees of the field fainted over him. 16 From the sound of his fall, the nations were shaken when I cast him down to Hades, together with those who descend into the pit. All the trees of splendor and the chosen plants of Lebanon, all that drink water, comforted him in the earth. 17 Yet they also went down to Hades with him among those slain by the sword, along with their posterity. They that dwelt under his shadow perished in the midst of their life. 18 To whom are you likened? Go down and be humiliated with the trees of splendor to the depths of the earth. You will sleep in the midst of the uncircumcised, with those slain by the sword. Thus shall it be for Pharaoh and the multitude of his strength,’ says the Lord and Master.”
(Ezekiel 31:13-18 OSB)
It seems as if GOD was saying here that Assyria was destroyed in the flood waters (v.15), along with those who benefited from Assyria's kingdom, while they were alive ("... perished in the midst of their life" v.17).
Some time later, GOD revealed more prophecy to Ezekiel in judgment of Egypt. GOD picked up with this Assyria theme, saying:
17 Again in the first month of the twelfth year, on the fifteenth day of the month, the word of the Lord came to me, saying, 18 “Son of man, lament over the strength of Egypt, for the nations will cast down her daughters dead into the depth of the earth, to those who go down into the pit. 19 They shall fall with him among those slain by the sword, and all his strength will sleep in death. The mighty men will say to you, 20 ‘Be in the depth of the pit, for to whom are you superior? Go down and sleep in death with the uncircumcised among those slain by the sword.’
21 “Assyria is there and all its company, and all the slain were put there. Their burial-place is in the depth of the pit, and his company are set round about his grave. 22 All the slain who fell by the sword are those who caused their fear to be upon the land of the living.
23 “There is Elam and all his army all around his grave, and all the slain who fell by the sword, and the uncircumcised who went down into the depth of the earth, and who caused their terror to be in the land of the living. Now they bear their torment with those who go down to the pit among the slain.
24“Meshech and Tubal and all their strength were put there around about his grave. All his slain men, all the uncircumcised slain by the sword, who spread their terror in the land of the living, are there. 25 They sleep in death with the mighty men who fell of old, who went down to Hades with their weapons of war. 26 Their swords were put under their heads, but their lawlessness was placed on their bones, because they terrified the mighty men in the land of the living. 27 As for you, you will sleep in death among the uncircumcised, with those slain by the sword.
28 “There are the rulers of Assyria who yielded his strength to a wound of the sword. 29 They sleep in death with the slain, with those who go down into the pit. 30 There are the rulers of the north, all the commanders of Assyria, who, with their terror and their strength, go down slain into the pit. They fell asleep uncircumcised with those slain by the sword, and they bear their torment with those who go down to the pit.
(Ezekiel 32:17-30 OSB)
Here, GOD repeatedly referenced the mighty men of old. These would be those mentioned in Genesis 6, including the giant (Nephilim) half-angelic sons.
Note how these mighty men are described as spreading terror on the earth.
As we will see, this "giant gene," if you will, got passed down to subsequent generations. Apparently, the wife of Noah's son Ham carried the "giant gene," because all of the giants (Nephilim) mentioned after the flood were descendants of Ham, primarily through Canaan.
(We know that Noah and his wife were free of angel-corrupted DNA because Jesus, the Messiah, had to be fully human and Jesus descended through Noah's son Shem.)
In order to properly appreciate why GOD chose to cleanse the earth and start over, we must realize the significant role that these sinning "sons of God" (angels) played and their offspring, the mighty men of old, the original Nephilim.