Because they mixed with Cains line. Cain was cursed. We all know when people get married one person can lead another astray. Many of Seth's lines (the only ones seen with the favor with God), started mixing with Cain and sure, assume other lines that also found no favor with God.
It was the rampant sin amongst the people of the earth because of or coupled with the sons of God and the nephilim that caused their death. Noah preached salvation for 100 years and no one believed him. All the lines were doomed to death if they did not believe - Seth's, Cain's, the others - all of them.
The ONLY one of Seth's line that was saved was Noah, and by extension his family.
Also, Cain was cursed that the earth would not be good to him, but we do not know whether or not he was forgiven or if he still worshiped God. We've all sinned and what Cain did or his relationship to God for that one sin is no different than your child lying about taking a candy bar.
sons of God are the angels (see also Job 1:6 or 38:7)
the nephilim were the spawn of the sons of God mating with human women; the author is stating that all the old tales and legends about Greek/Roman/Egyptian/Sumerian gods were these people
Son of God IN THE OLD TESTAMENT The title "son of God" is frequent in the Old Testament. The word "son" was employed among the Semites to signify not only filiation but other close connexion or intimate relationships. So, "a son of strength" was a hero, a warrior, "son of wickedness" a wicked man, "sons of pride" wild beasts, "son of possession" a possessor, "son of pledging" a hostage, "son of lightning" a swift bird, "son of death" one doomed to death, "son of a bow" an arrow, "son of Belial" a wicked man, "sons of prophets" disciples of prophets, etc.
The title "son of God" was applied in the Old Testament to persons having any special relationship with God. Angels, just and pious men, the descendants of Seth, were called "sons of God" (Job, i, 6; ii, 1; Ps. lxxxviii, 7; Wisd., ii, 13; etc.). In a similar manner, it was given to Israelites (Deut., xiv, l); and of Israel, as a nation, we read: "And thou shalt say to him: Thus saith the Lord: Israel is my son, my firstborn. I have said to thee: Let my son go, that he may serve me" (Ex., iv, 22 sq.). The leaders of the people, kings, princes, and judges, holding authority from God, were called sons of God. The theocratic king as lieutenant of God, and especially when he was providentially selected to be a type of the Messias, was honored with the title "Son of God". But the Messias, the Chosen One, the Elect of God, was called the Son of God (Ps. ii, 7). Even Wellhausen admits that Ps. ii is Messianic (see Hast., Dict. the Bible", lV, 571).
So Son would mean an intimate relationship with whatever they are the son of. Means that it's a title that can be given to any number of things, it doesn't have to be angels and doesn't have to be humans, it doesn't even have to be male.
Nephilim, a Hebrew word often rendered as “giants,” may also mean “fallen ones.” However, given that angels do not have bodies, which are needed for the procreation of human children—barring a miracle—and given that in heaven human beings live like angels and thus do not get married (Matt. 22:30), the identity of the “sons of God” points to mere humans. Cain, the immoral son of Adam. Thus, “fallen ones” Nephilim can also be translated as “tyrants,” violent men who want to make a name for themselves and will do whatever it takes to achieve it. Which makes sense if they were immoral "fallen ones" from Cain.
Because they mixed with Cains line. Cain was cursed. We all know when people get married one person can lead another astray. Many of Seth's lines (the only ones seen with the favor with God), started mixing with Cain and sure, assume other lines that also found no favor with God.
It was the rampant sin amongst the people of the earth because of or coupled with the sons of God and the nephilim that caused their death. Noah preached salvation for 100 years and no one believed him. All the lines were doomed to death if they did not believe - Seth's, Cain's, the others - all of them.
The ONLY one of Seth's line that was saved was Noah, and by extension his family.
Also, Cain was cursed that the earth would not be good to him, but we do not know whether or not he was forgiven or if he still worshiped God. We've all sinned and what Cain did or his relationship to God for that one sin is no different than your child lying about taking a candy bar.
Some notes popped up while reading what you wrote.
sons of God are the angels (see also Job 1:6 or 38:7)
the nephilim were the spawn of the sons of God mating with human women; the author is stating that all the old tales and legends about Greek/Roman/Egyptian/Sumerian gods were these people
Yes, I agree Cain was cursed and marked