Reference: Pitterson, Ryan. Judgment of the Nephilim. Days of Noe Publishing: New York, NY. 2017.
Beginning in the Garden of Eden a War of the World has been fought for thousands of years. "Humanity is in a great war between God and His allies, righteous angels and saints, and the Devil and his army of fallen angels, demons and human agents." (Introduction). God told Lucifer (Satan) that one day there would be birthed a male child that would overcome Satan and bring to an end his armies arrayed against humanity. Humanity would have the opportunity of redemption by what Jesus' death on the Cross proclaimed. Ever since, Satan has worked diligently to thwart God's plan
Satan convinced 200 angels to break off fidelity to God, enter earth, and mate with humans as a major attempt to cutoff and prevent God's plan for humanity to be fulfilled. They were termed "the sons of God." The fallen angels took human wives, the "daughters of men." These "hybrid half -human, half-angels became known as the Nephilim." (Introduction). This was an attempt by Satan to attack the human gene pool and to corrupt humanity's gene pool/DNA. If Satan were to have succeeded in this undertaking, there would be no Jesus and no salvation for mankind. It would be Check Mate by Satan. God would not and could not allow Satan to succeed.
It is critical we be ever mindful of what is written in Ephesians 6:11-12. "Put on the full armor of God, that ye may be able to stand against the wiles of the devil. For we wrestle not against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this world, against spiritual wickedness in high places." Humanity in the pre-Flood world did not have the benefit of scripture such as this. There was an invasion by these angels and their creation, the giants that have been named Nephilim. The Nephilim basically became monsters.
The Book of Enoch indicates, "...the whole earth has thereby been filled with blood and unrighteousness." [Enoch 9:9] "And the angels taught them charms and spells,...And the women became pregnant, and they bare large giants,...The giants consumed all the work and toil of men. And when men could no longer sustain them, the giants turned against them and devoured mankind." [Enoch: 7, 2,4].
Very simply stated, the abominations of the Nephilim caused God to start all over by causing Noah to build the Ark and collect the animals and take off with his three sons and their wives on the Ark. The earth was covered with water and humanity started all over again.