"The Nephilim were on the earth in those days - and also afterward - when the sons of God went to the daughters of men and had children by them. They were the heroes of old, men of renown." NIV translation
Who were the Nephilim? The word actually comes from "napha" which means fallen. There are those who say they were aliens or fallen angels but what it actually is talking about is humans that have totally fallen from God and have been turned over to Satan. As we have seen, the "sons of God" refers to people that were following God which is the line of Seth and the "daughters of men" refers to the daughters of Cain's descendants who after a few generations were totally turned over to Satan. The Nephilim were the children of these unholy marriages. We see that these children even became leaders of the world as they were known as "men of renown".
"The Lord saw how great man's wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time." NIV translation
These "sons of God" probably did not intend to turn their backs on God and they may have even thought that they could bring these women to the Lord. Instead, what happened was these women led the men away from God and into idolatry, especially what we would call today "humanism". Like Cain's descendants, these "sons of God" and their children went into a downward spiral until they were completely controlled by sin and Satan.
"The Lord was grieved that he had made man on the earth, and his heart was filled with pain." NIV translation
Just as we mourn at the death of a loved one, God was mourning the sad state of His creation. His heart ached for the people and all of the creatures that were under man's care. Just as a parent is filled with sorrow when they see their child going down the wrong path, God was pained to watch His children not only walking down the wrong path but even running in the wrong direction.
"So the Lord said, 'I will wipe mankind, whom I have created, from the face of the earth - men and animals, and creatures that move along the ground, and birds of the air - for I am grieved that I have made them.'" NIV translation
God is love and with the deepest of love comes the dark side of love which is jealousy (see Zephaniah). Man was created for God and He could not allow this complete disregard for Him to continue.