If the word Nephelim has any connotations of 'Fallen', then that word must be taken in the context of all the other ways the word 'Fallen' is used in this context. It would be obvious to say, that wherever this word is used, it has reference to sin. The two words are synonymous in that respect. Satan fell from his position. Luke 10:18 'Then I saw Satan fall from heaven'.Jude 1: 6-8 States quite clearly That the fallen Angels left their first estate..
New American Standard Bible (NASB)
6 And angels who did not keep their own domain, but abandoned their proper abode, He has kept in eternal bonds under darkness for the judgment of the great day, 7 just as Sodom and Gomorrah and the cities around them, since they in the same way as these indulged in gross immorality and went after [a]strange flesh, are exhibited as an [b]example in undergoing the punishment of eternal fire.
8 Yet in the same way these men, also by dreaming, defile the flesh, and reject authority, and revile [c]angelic majesties.
They certainly fell from heaven to Earth. Enoch Says specifically that 200 Angels with Satan at their head, chose a path, to come down into the daughters of men.
I think it is to easy to rationalize these issues. The truth is Pre flood history is well hidden. Of course we can glean many things from works such as the Gilgamesh epic and other Sumerian Babylonian etc histories, but sadly these are clouded by fallen interpretations. Never the less we have adequate information laid before us to understand the true history of the World in the word of God, we just need the wisdom from God to interpret it correctly. Lets not make the mistake of interpreting what seems like fantasy as fallacy. No one can deny that the, 'result', of the Sons of God, entering into the daughters of men, was a race of unnatural beings. This could only have occurred, if there was an addition, to the human physical genome. Fallen angels co-habiting with the daughters of men, could certainly have caused this. That there is absolutely no context for, 'Space men', is obvious from the Whole context of the bible.
According to Genesis 6:4, the Nephelim existed on the Earth before they came into the daughters of men.
Genesis 6:4 (NASB)
The Nephilim were on the earth in those days—and also afterward—when the sons of God went to the daughters of humans and had children by them. They were the heroes of old, men of renown.
It would be strange, to say then that man, or a Holy line of men, existed on the Earth, before they went into the daughters of men, this would be obvious. That fallen angels are acting in the world to effect their evil intent, is clear through the whole history of the world. Would it be radical then to say at a point after the creation and fall of man that these fallen angels were permitted to assume the form of flesh and mate with the daughters of men. Not when we consider the fact, that Satan himself, the chief fallen angel, appeared as a serpent, to deceive Eve. Once Adam and eve fell we know sin had a ravaging effect, even to the extent that Cain murdered Abel simply because he was 'cast down'. After the fall, Satan was permitted to 'run wild' as is his nature. Going to far in his madness he planned this scheme to corrupt the seed of God. That seed that would be the line that the Messiah would come from. Satans purpose through history has been simply to thwart Gods purpose because he knows that if that purpose is fulfilled, he is finished. Introducing a genetic modification into mankind would have been a great plan from his point of view....To corrupt the natural line of genetics with his own poison, would result in turmoil on Earth and so it did. The resultant offspring of giants were a major source of suffering for mankind. Gods answer to this action by Satan and the daughters of men, was to declare the end of mankind by a flood. This was a sure way of re setting as well as continuing the history of redemption in Christ Jesus..the seed of God..as opposed to the seed of Satan. Satan Im sure was privy to the conversation in the garden of Eden. He knew Gods plan was to introduce a seed that was pure and powerful. A seed that would bring about his destruction..
New American Standard Bible (NASB)
15 And I will put enmity
Between you and the woman,
And between your seed and her seed;
He shall [a]bruise you on the head,
And you shall bruise him on the heel.”
Satan it appears, in all his malice, considered how he could wipe out the line of the seed that would crush him. Introducing a genetic pollutant, into the race of mankind, was the route he took. The result was chaos. Giants, according to Enoch, who devoured the Earth of all food. When they were not satisfied by what God had provided for mankind, they turned and ' Sinned against animals and birds' (by eating them)..when not satisfied even with that, they turned on man and devoured him also. This Enoch describes explicitly.
It makes complete sense when taken in conjunction with Genesis 6. Gods judgement on this mating was to completely destroy mankind....If the judgement meets the crime, then the crime could have been devastating to the Holy line that would bear the Messiah...and there was need of a radical move to safeguard the Holy seed from the genetic pollution of fallen angels. The flood.
We must not divorce, verses 1-5 from verses 5-8. The Nephelim greatly increased evil on the earth, both by mating with the daughters of men and by teaching mankind all manner of corruption and rebellion against God. The Judgement was as great as the sin and therefore we must conclude the sin to have been great. Granted Holy men marrying unholy women would have been a terrible sin also. But a union of Holy men and sinful daughters would not have created a genetic anomaly under normal circumstances. This taken with the fact that these Nephelim were on the Earth before they mated with the daughters of men gives strong warrant for their being 'Fallen' angels walking on the Earth. Of course they were men of renown. They were angelic beings and had knowledge that surpassed that given to man. They had been in the heavenly places and had gifts many, gifts which were perverted to Satan's evil schemes. This Enoch also states when he says that 'They taught men the art of war' and other things. Naturally Satan insinuated his poison, as far as he could into mankind. After all He had declared war on God and would teach man how to do the same thing. To some degree this consists in man killing man. Thou shalt not kill is the law of God. When we kill, we fight against God by doing so.! Satan surely taught man war . Enoch explains that on a physical level, where man was taught how to make weapons and armor.
New American Standard Bible (NASB)
The Corruption of Mankind
6 Now it came about, when men began to multiply on the face of the land, and daughters were born to them, 2 that the sons of God saw that the daughters of men were [a]beautiful; and they took wives for themselves, whomever they chose. 3 Then the Lord said, “My Spirit shall not [b]strive with man forever, [c]because he also is flesh; [d]nevertheless his days shall be one hundred and twenty years.” 4 The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown.
5 Then the Lord saw that the wickedness of man was great on the earth, and that every intent of the thoughts of his heart was only evil continually. 6 The Lord was sorry that He had made man on the earth, and He was grieved [e]in His heart. 7 The Lord said, “I will blot out man whom I have created from the face of the land, from man to animals to creeping things and to birds of the [f]sky; for I am sorry that I have made them.” 8 But Noah found favor in the eyes of the Lord.