Genesis 3:15 And I will put enmity between you and the woman, and between your offspring and hers; he will crush your head, and you will strike his heel
The Garden Of Eden Revisited
In order to understand the reason for the flood of Noah, I believe it’s rather important to first start in the passage above. This pronouncement from God to the serpent (the nachash i. e. Satan) and Eve, is missed by many people. When Eve was beguiled by the nachash, by eating the forbidden fruit, this set and wound the clock towards the eradication of all evil in the universe. People generally assert that evil entered into the world with Adam and Eve disobeying God, but don’t usually recognize that a deceiver  was already moving freely throughout the garden before they fell! It is not the purpose of this article to describe all the effects of the fall in the garden, but one major change was that sin entered into the DNA of mankind. And at that point, they/we needed a savior.
We can see that there were to be offspring of both the woman and the serpent. Most translations render offspring (Hebrew: zera‛) as seed. What’s usually accepted as the seed of the woman is the person of Christ, this being the first mention of the Proto-evangelium.  The seed of the serpent is almost always taught that it’s a figurative seed of Satan, being the seed of evil. While I do agree that this seed is metaphorical in one sense -as we have just mentioned- but I also believe that this seed is literal as well! It’s my position that, Satan knew very well that the redemption of man would come through the lineage of man, and he was to stop that at all cost. I do want to make a distinction here, while I believe that he does have a seed, I don’t subscribe to the “serpent seed theory” .
Fallen Angels Or Godly Humans?
(Emphasis mine) Genesis 6:1-4 When the number of people started to increase throughout the fertile land, daughters were born to them. The divine beings (Hebrew: b’nai ha Elohim) saw how beautiful these human women were, so they married the ones they chose. The Lord said, “My breath will not remain in humans forever, because they are flesh. They will live one hundred twenty years.” In those days, giants (Nephilim) lived on the earth and also afterward, when divine beings and human daughters had sexual relations and gave birth to children. These were the ancient heroes, famous men.
This passage in Genesis six is one that has been a source of controversy for hundreds of years within the church. Without going into too much detail, where this version of the Bible (Common English Bible) mentions divine beings, most translations render the b’nai ha Elohim merely as Sons Of God. The oldest view of who these “sons” were, is that they were angels, more precisely fallen angels. They were also known as the Gregori or The Watchers. There are two other views which assert that these sons were human, from a Godly line of Seth or human rulers. I’ve addressed some of these views and their objections to the “angel view” previously. One of the more telling factors in the understanding of this scripture is that the Codex Alexandrinus  (Codex A) renders them as hoi angeloi tou Theou which translate as “The Angels Of God”.  There are other ancient manuscripts -such as Enoch, Jubilees, and Jasher  etc- that clearly define who these beings were and mentions their various atrocities (such as genetically modifying the plant life and creating chimeras/hybrids) that they did. Such phenomena are rather bizarre, but antiquity is full of accounts like this; the Mayans, Incas, Aztecs, Indians, Chinese, Romans, Greeks, British, Native Americans etc all speak of such things. Even Plutarch records a Roman army running into a living breathing Satyr.
(Emphasis mine) Genesis 6:9-13 These are the generations (Hebrew: tôledâh) of Noah: Noah was a just man and perfect in his generations (Hebrew: dôr), and Noah walked with God. And Noah begat three sons, Shem, Ham, and Japheth. The earth also was corrupt before God, and the earth was filled with violence. And God looked upon the earth, and, behold, it was corrupt; for all flesh had corrupted his way upon the earth. And God said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth.
Now with getting to the brass tax of the reason for the flood, we must consider why Noah’s “generations” were a specific point of interest in the above passage. The Bible mentions here that he was “perfect in his generations”, but what does that mean? It’s talking about his DNA or gene pool! We know that Noah and his family were righteous followers of God, but in addition to that, their bloodline hadn’t been tainted by the the seed of the fallen ones, the Nephilim. Indeed the Nephilim did do evil in the sight of the Lord, and were a rebellious group but there’s much more to the story. From a take (in the reverse) of the Euthyphro Dilemma, we can ask, were the fallen ones evil because they did evil, or did they do evil because they were the fallen ones? The answer is yes. They were intrinsically evil demonic hybrids that had nonredeemable value, and thus were eliminated. It was for this reason that God sent the flood, as none of these individuals could be rescued. This eradication of such beings -we do see a second incursion after the flood, but not as widespread- was crucial, as God was preserving the gene pool of mankind, keeping it pure, so a messiah (Jesus) could come and be a kinsman redeemer. 
Note: I am barely skimming the surface on this topic -there are more articles to come- of what was actually going on here, but I’m trying to keep this article short. We see in other places in scripture that these creatures were called “the dead” (Hebrew: Rephaim) that “will not rise” , meaning that they won’t have a resurrection. These were no ordinary humans. (Here’s an interesting video from Semitic scholar Dr. Michael Heiser that gives us way more detail than what I have written here)
1. The verb form of nachash means to “deceive or to divine = practice of divination”. For this being to deceive Eve, he himself would have already been fallen beforehand.
2. What is protoevangelium? http://wiki.answers.com/Q/What_is_protoevangelium
3. Serpent seed – http://en.wikipedia.org/wiki/Serpent_seed
4. This manuscript is considered rather important: “The text of Codex A is considered one of the most valuable witnesses to the Septuagint.” http://www.newadvent.org/cathen/04080c.htm
5. Who Were the “Sons of God” and the “Giants” in Genesis 6:1–4? http://www.struggler.org/Giants.html
6. Starting on page 16 we see some interesting writings from Enoch, Jubilees, and other such writings that expound on what was happening in Genesis: Grace Theological Journal 5,1 (1984) 13-36
7. Our Kinsman Redeemer http://www.abideinchrist.com/messages/lev25v25.html
8. (Emphasis mine) King James Version Isaiah 26:14 They are dead (Hebrew: mûth), they shall not live; they are deceased (Hebrew: rephaim), they shall not rise: therefore hast thou visited and destroyed them, and made all their memory to perish.
If you read the Douay-Rheims Bible translation of this scripture in Isaiah, you’ll see that they call these dead, the giants! They hit the nail on the head with this and understood the distinction of the deceased.