In this 22nd video of the book of Genesis Dr Woodhead discusses the importance for us to to make two observations.
1. Wickedness of man was great on the earth
2. Every imagination of the thoughts of his heart was only evil continually
The wickedness from the line of Cain as well as the progeny of the fallen angels and promiscuous women produced people who were unrestrained in their wickedness. This is a description of their outward sinful behavior. Since our thoughts drive our behavior the second observation was descriptive regarding the degree of evil they were thinking. The Hebrew word for imagination is yatzar. It is the same word used in describing God’s formation of man. God formed mankind by design but humanity took its God-given abilities (design) and devised evil continuously.
Proverbs 23:7 For as he thinketh within himself, so is he: Eat and drink, saith he to thee; But his heart is not with thee (ASV 1901).