Are today's Demons, the "sons of God" killed in the flood?

Anyone that has read the Bible at all knows it speaks of people being possessed by "devil(s)" or "unclean spirits." Christ and His disciples dealt with possessed people regularly and always cast the invading spirits out (Matt. 4:24; Mark 6:7, etc.). As to how these spirits originated or became evil, the Bible is mostly silent. There is some indication they may be the spirits of the "sons of God" who took on flesh and came unto the "daughters of men" in Noah's day and later died in the flood (Gen. 6:1-4), but there is no proof that is the case. Others suggest they are angels that chose to rebel with Lucifer (Isa. 14; Eze. 28) before man's creation and subsequently became "devils," but there is no clear proof of this either.