The reason 200 watcher angels led by Samjeya and Azazel , were behind the rebellion, was that they were jealous of mankind. The angels were non sexual and could not reproduce. So they came down to Mt Hermon and went to the Sumerians, Egyptians, ancient kings and said."we will be your gods if you give us your women as wives in exchange for our great knowledge." Men were given the earth to rule by Yahweh so they had to give these beings permission to do this, the angels could not force it. These watchers existed in the spiritual realm so they had to leave that form and become as men and functioned sexually but many were huge and they gave birth to a race of giants called Nephilim. Some of their sons and daughters were up to a mile high- the titans of old ,the bible called them, "men of renown", which make up many mountain ranges we see today. "The rocks are alive". That's why the flood waters remained high & on the earth for months, to drown them all and removed all corrupted DNA.