The Bible says it repented God that He had made man on the earth,and it grieved Him at His heart.
This happened after the sons of God came unto the daughters of men,and they bare children to them.
The children became mighty men which were of old,men of renown.
These children because of that breeding were terribly wicked in that they were continuously evil,which means they were so evil that all they did was evil with no regard for God.
After that breeding they were not the type of people that God intended and they could not continue to be on earth for then God's ways would never be seen,so God caused a flood to be on earth to take them away.
That breeding caused terrible wickedness to where they could not do good and God's plan could not continue.
Some people believe that it was fallen angels that corrupted mankind,for it does not make sense that they were that wicked by normal human to human interaction,for why were they not like that before,but like that after the sons of God came unto the daughters of men,and if they were normal humans then those sons of God would have already interacted with women before that time.
Also if they were sons of God referring to normal humans,they would not be wicked to take as many wives as they wanted.
Jesus called Judas a devil,and the Bible calls him the son of perdition,who went to his own place,which the beast who is the devil inhabiting a human body,goes to his own place where no one goes.
The Bible says that the devils that left their own habitation are reserved in chains under darkness until judgment,and the man who was called legion because he had many devils,the devils begged Jesus not to send them to the deep or bottomless pit,for they fear the bottomless pit where the fallen angels that are in chains are at.
Jesus said the coming of Him will be as the days of Noah and the 5th trumpet opens up the bottomless pit which seems to indicate that the fallen angels in chains are let loose to interact with mankind to cause all people who do not love God to be continuously evil again so God can judge them and put them down.
Like in the days of Noah where the fallen angels interacted with mankind they will interact with them again causing the beast system,in which the Bible says the earth has become the habitation of devils.
It seems like that is the only conclusion is that the fallen angels interacted with mankind and caused them to be continuously evil in which mankind was not in their normal decision making capacity because the devils corrupted them.
The earth was filled with violence in those days and even though Noah and his family were not corrupted the people would of eventually took them out leaving no one to regard God,nor could they regard God because of the devils interacting with them,so God caused a flood and saved Noah and his family to start over.
After the flood God did not allow the fallen angels to interfere with mankind,and the fallen angels that were not in chains feared that place,but the fallen angels in chains will be let loose on mankind to deceive all who love not God,which is the strong delusion,a delusion that they cannot snap out of but will follow the antichrist,like in the days of Noah where they were in strong delusion and could not follow God but were evil continuously.
If the sons of God are not fallen angels it does not make sense that they would become like that evil continuously if it were normal human to human interaction,because why were they not like that before,and why would they all of a sudden be like that.