It was the purpose of Satan and his fallen angels to corrupt the human race and thereby do away with pure Adamite stock through whom the seed of the woman should come. This would avert their own doom and make it possible for Satan and his kingdom to keep control of the planet earth indefinitely. It was said to Adam and Eve that the seed of the woman should defeat Satan and restore man’s dominion.
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.
The only way then for Satan to avoid this predicted defeat was to corrupt the pure Adamite line so that the coming of the seed of the woman into the world would be make impossible. This, he tried to accomplish by sending some of his fallen angels to marry the daughters of men as in,
And it came to pass, when men began to multiply on the face of the earth, and daughters were born unto them,
That the sons of God saw the daughters of men that they were fair; and they took them wives of all which they choose.
And the Lord said, My spirit shall not always strive with man, for that he also is flesh: yet his days shall be an hundred and twenty years.
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.
. . .and producing the giant nations through them.
There are two such eruptions of fallen angels taught in Gen. 6:4: There were giants in the earth in those days (before the flood); and also after that (after the flood), when the sons of God (fallen angels) came in unto the daughters of men (any daughters of men — Cain, Seth and others), and they bare children to them (to the angels).
Satan almost succeeded in his plan during the first eruption, for all flesh had corrupted his way upon the earth; and of all the multitudes Noah and his sons were the only pure Adamites left to be preserved by the ark. (Click to Article)