Kabbalah su Genesi 6:2

Rabbi Chiya said: For 300 years before the Great Flood, Noah warned them to change their ways but they did not listen to him until the time when the Holy One, blessed be He, had finished waiting for them to repent. Then they were lost from the world. Come and behold, it is written: "And it came to pass, when men began to multiply on the face of the earth, and daughters were born to them" (Ibid. 1), who they went naked in front of everyone. And then, what is written? "And the sons of Elohim saw the daughters of men (Ibid. 2). This was the main cause to continue sinning until it finally caused them to be destroyed. And because of that they followed the Evil Inclination, held fast to its trunk and roots, rejected the holy faith from among themselves, and became defiled. So it is written: "The End of all Flesh is come before Me" (Ibid. 13), to teach that they were guilty.

It says in Beresheet Rabbah: Rabbi Simlai said: Wherever you find lust, an epidemic visits the world and slays both good and bad. Rabbi Azariah and Rabbi Yehuda son of Rabbi Simone said in the name of Rabbi Yehoshua ben Levi: The Creator is long-suffering for everything besides immorality, as it says “The sons of God saw the daughters of men” (Genesis 6:2) which is followed by, “And the Lord said I will blot out man (Genesis 6:8).” Rabbi Yehoshua ben Levi said in Rabbi Pedaya’s name: That whole night Lot prayed for mercy for the Sodomites, and they [the angels] would have heeded him, but as soon as they [the Sodomites] demanded “Bring them out unto us, that we may know them (Genesis 19:5)—for intercourse— they [the angels] said, “Who else is here” (Genesis 19:12) meaning, to plead in their defense, from now on you are no longer permitted to plead in their defense.

About the Nefilim (the fallen ones), it is written: "That the sons of the Elohim saw that the daughters of men were fair" (Beresheet 6:2). These are the second group (of the erev rav, emanating from the Chochmah of the Kelipot) from those fallen from above, (from Aza and Azael, who were angels above. Hashem dropped them out of the heavens). When the Holy One, blessed be He, desired to create man, he said (to the angels) "Let us make man in our image..." (Beresheet 1:26). He wanted to make him a leader over all (the angels above), so that he might govern all (the angels) and they would be under his rule, as is written about Joseph: "And let him appoint officers over the land" (Beresheet 41:34).