BBC: Genesis 6:4-6
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. 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. And the Lord regretted that he had made man on the earth, and it grieved him to his heart. – Genesis 6:4-6
Part of the reason for the misconstruction of the previous verses (Gen 6:1-3) is because of the word “Nephilim” and the description of the great men of old. However, the text here affirms what is stated elsewhere (Num 13:33) which is that the Nephilim upon the earth both before and after the flood. The clear understanding of the text does not permit Noah nor his sons to be offspring of the Nephilim. The Nephilim are simply giants.
The reason for the flood is not angelic beings. It is the perversion of “man”. Man’s wickedness is so spread across the earth that is has even corrupted the line by which God means to bring about His promise to Eve (Gen 3:15).
Many words have been spent looking for a comprehensive thought to “God repented”. What need be said is simple, God subjected man to the judgment that was required. God submitted to the judgment His nature and character required Him to bring. This brings sadness to Him because all of creation is to bring Him glory. Through creation’s seed creation was to be redeemed. God must elect a new man to save creation.