Genesis 6 describes the ever increasing cooperation between men and demons. Demonic activity before the flood was on the rise. Not only did men grow in their wickedness, but they chose to give heed to the voice of Satan and demons rather than the voice of God. They had no excuse. They rejected God. They rejected Enoch's preaching (Jude 14). They rejected Noah's preaching (2 Peter 2:5). As Noah grew in age, the number of righteous men steadily decreased. There were so few righteous men on the earth, that 120 years from the flood 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. (Genesis 6:5)
The only man who found favor in the eyes of the LORD was Noah (Genesis 6:8). This alone is a shocking statement. Out of the millions upon millions (and possibly billions) of people who lived on the face of the earth, only Noah remained faithful. Only Noah sought after God. The rest of the world continued on a path to destruction. However you interpret the first four verses of Genesis 6, the purpose of these verses is the same— that despite God's plea, men continued to reject God. They followed in Cain's pattern of refusing repentance. They did not reject Lamech's gross immorality; instead, they embraced it.
God is patient with men and gives them the opportunity to return to Him, but there also comes a time when He is no longer patient. There comes a time when God's mercy comes to an end. Those who lived before the flood were given 120 more years. For 120 years, God would strive with men. For 120 years, men would have the opportunity to repent and live out a life for the glory of God. But the men in the pre-flood world did not repent— and so God sent the flood— for His Spirit does not strive with men forever (Genesis 6:3).
The purpose of Genesis 6:1-4 is to point out that today is the day to turn to God— that today is the day for repentance, for a day is coming when God's patience will end. The great tragedy of Genesis 6 is that out of the millions (or billions) of people on the planet, only eight were saved. Only eight rejected the rampant wickedness in the world and stood for the truth.
120 More Years of Mercy - Genesis 6:3
Cain and Abel: Sarcastic Reply - Genesis 4:9
Lamech's Song - Genesis 4:23-24
What Does the Bible Say About Polygamy? (Part 1) - Genesis 4:19
State of the World (2948 BC): Noah's Birth - Genesis 5:28