I seem to recall from the Jeff Cavins Bible study series “Great Adventure Bible Study”, Jeff mentioned something about one group of people (the Giants I think) were the decendents of Cain
who were more corrupt than the decendents of Seth &/or other children of Adam.
However, the footnotes in the New American Bible (below) talk of the adoption of old legend.
Footnotes
1 [1-4] This is apparently a fragment of an old legend that had borrowed much from ancient mythology. The sacred author incorporates it here, not only in order to account for the prehistoric giants of Palestine, whom the Israelites called the Nephilim, but also to introduce the story of the flood with a moral orientation–the constantly increasing wickedness of mankind.
2 [2] The sons of heaven: literally “the sons of the gods” or “the sons of God,” i.e., the celestial beings of mythology.
3 [3] My spirit: the breath of life referred to in
Genesis 2:7. His days . . . years: probably the time God would still let men live on earth before destroying them with the flood, rather than the maximum span of life God would allot to individual men in the future.
4 [4] As well as later: According to
Numbers 13:33, when the Israelites invaded Palestine and found there the tall aboriginal Anakim, they likened them to the Nephilim; cf
Deut 2:10-11. Perhaps the huge megalithic structures in Palestine were thought to have been built by a race of giants, whose superhuman strength was attributed to semi-divine origin. The heroes of old: the legendary worthies of ancient mythology.
To me personally, the only instructional aspect of this is what Jeff says.
Jim