‘Yahweh saw that the wickedness of man was great in the world (or “in the land”), and that every imagination of the thoughts of his heart was only evil continually.'
The occult activity, which had clearly become commonplace, emphasised the depths to which man had sunk, and it is quite clear that the menfolk had connived in it. Indeed without the illustration of verses 1-4 this description and what follows would be inexplicable.
In the past men have murdered their kinsmen, and others, and have been spared, revealing God' compassion and mercy. Thus something particularly awful was required to bring about what was to happen. These humans are judged to have become totally caught up in evil, and that includes the surviving sons and daughters of Lamech, and possibly even of Methuselah. Indeed he might himself have died in the flood. The description is very emphatic. Every imagination of the thought of the heart continually evil. This is not just man sinning, it is a great deal more than that. There is no goodness, no compassion, no altruism, no thoughtfulness, no unselfishness, no genuine love, nothing that makes life wholesome. Satanic possession has indeed gripped the land.
Notice the contrast between Genesis 1:31 where ‘God saw all that he had made and it was very good' with these verses ‘Yahweh saw that the wickedness of man was great in the earth --- and was sorry that he had made man on the earth'. The creation was good, but once man took over it sank to this.