The Jewish Virtual Library provides a quotation from the Encyclopedia Judaica on the Nephilim which states:
NEPHILIM (Heb. נְפִילִים), a race of giants…Genesis 6:1-2 relates that the “sons of gods,” i.e., divine or angelic beings, took mortal wives…[this] follows an ancient tradition in equating the Nephilim and the gibborim as offspring of the union of angels and mortals…The angels were then depicted as rebels against God: lured by the charms of women, they “fell” (Heb, nfl. נפל)…and introduced all manner of sinfulness to earth.
Their giant offspring were wicked and violent; the Flood was occasioned by their sinfulness. (None of these ideas is in the biblical text.)
Note the reference to the Angelic view and that equating the Nephilim and Gibborim as being the “ancient tradition” which indeed it is: ancient Jewish and early Christian.
However, the assertion that 1) this ancient tradition and 2) that the Angels were rebellious and 3) introduced manners of sinfulness and 4) that the giants were wicked and violent and that 5) the Flood was occasioned by their sinfulness, etc. are not in the biblical text is overreaching.
Let us elucidate: 1) The ancient tradition is within the biblical text’s New Testament, which the Encyclopedia Judaica may bypass on this point. 2) This is not stated by Genesis 6 but may be implied.
3) This is also not stated in Genesis 6; it is elucidated in later apocryphal texts such as the Book of Enoch.
4) This may be too specific a claim, what the Bible does state about them is that they “became mighty men which were of old, men of renown.” This claim is related to the next one.
5) Genesis 6 follows a narrative from the Sons of God and daughters of men affair, to the Nephilim/Gibborim, to their renown with the very next verse, after declaring their renown, being, “And GOD saw that the wickedness of man was great in the earth, and that every imagination of the thoughts of his heart was only evil continually” which leads directly to the account of the flood.
The Jewish Virtual Library / Encyclopedia Judaica continues by delving into the apocryphal traditions about
God decreeing that the Nephilim massacre each other. They also note that “evil spirits” aka demons “originally issued from the bodies of the slain giants.”
We get a taste of changing the “ancient traditions” when it is noted that “The ‘sons of God’ are explained in the Targum to Genesis 6:4 and the Midrash (Gen. R. 26:5) as young aristocrats who married the daughters of commoners. The Targum renders both gibborim and Nephilim by gibbaraya; the Midrash (Gen. R. 26:7) lists seven names applied to giants.”
An interesting statement is made that in “post-talmudic literature (cf. Rashi, Yoma 67b) the long-suppressed myth came to the surface again” (emphasis added for emphasis); suppressed by whom is not elucidated and may be a mere reference to losing and then regaining interpretive favor.
Being the paraphrase that it is, the Palestinian Targum replaces the term Nephilim in Genesis 6:4 as “Shamhazzai and Uzziel fell from heaven and were on earth in those days.” As an FYI: Midrash Genesis Rabbah has Shamchazai and Azael (or, Azazel) in place of Uzziel (and/or, Uzza and with Shamhazzai simply being one of many transliteration of that name). See Louis Ginzberg’s The Legends of the Jews on the Nephilim and Islam’s magickian fallen angels Harut and Marut for how these Angels are relevant.
The point is that the Nephilim are being identified with Angels which is also that which is done by Midrash Aggadat Bereshith, naming them Uzza and Uzziel, but this Midrash dates to the 10th century AD and is thus, hardly relevant to the issue of ancient traditions. This late Midrash also plays identifies the Nephilim as being humans of Cain’s lineage.