In the famous apocryphal that has not been integrated into the Catholic Church Canon, Enoch explains that the angels were created on the second day of creation and were assigned to various positions. One of the angels, called Satanel, rebelled because he thought he could become more powerful than God. God, therefore, threw him out of heaven.
The rebel angels leaded by Azazel are called “Watchers”, Egregorians or Beney Elohim in Hebrew. Among them, Asmodeus, Satanaël, Sammaël, Belial, Mastema, Milki-Resa and Semihazah
“When men began to increase on earth and daughters were born to them, the divine beings saw how beautiful the daughters of men were and took wives from among those that pleased them. The Lord said, ‘My breath shall not abide in man forever, since he too is flesh; let the days allowed him be one hundred and twenty years.’ It was then, and later too, that the Nephilim appeared on earth – when the divine beings cohabited with the daughters of men, who bore them offspring. They were the heroes of old, the men of renown.” – Genesis 6:1-4
This is the only account of the watchers in the Bible. According to some early Christian writers the mysterious episode of Genesis is the proof that the demons are the offspring of the supposed union of the angels of God and the daughters of men. Strangely, these theories are similar to the thesis supported by the apocryphal Books of Enoch found near Qumran (20 km from Jerusalem near the Dead Sea) in 1947. The first objection to the legend was, that the angels as pure spirits, were plainly incapable of feeling sensual passions.
The answer is lust, the angels were not overpowered with the passion of sensual love while they were as yet in their purely spiritual state; but when they looked down and witnessed the wickedness and ingratitude of men whose sins were defiling the fair creation of God, they asked of their Creator that they might be endowed with bodies like those of men, so that coming down to earth, they might set things right and lead a righteous life in the visible creation. Their wish was granted, they were clothed in bodies and came down to dwell on earth.
But now they found that with their raiment of mortal flesh they had acquired also the weakness and passions which had wrought such havoc in men, and they too, like the sons of men, became enamored of the beauty of women and, forgetting the noble purpose of their descent to earth, gave themselves up to the gratification of their lust, and so rushed headlong to their ruin.