(excerpt from gotquestions.org)
“Some claim that Scripture shows God having second thoughts about His creation: “The Lord saw how great the wickedness of the human race had become on the earth, and that every inclination of the thoughts of the human heart was only evil all the time. The Lord regretted that he had made human beings on the earth, and his heart was deeply troubled. So the Lord said, ‘I will wipe from the face of the earth the human race I have created—and with them the animals, the birds and the creatures that move along the ground—for I regret that I have made them’” (Genesis 6:5-7).

It’s good to understand the word regret in this passage. When used of God, regret incorporates the thought of compassionate grief and an action taken. God was not showing weakness, admitting an error, or regretting a mistake. Rather, He was expressing His need to take specific, drastic action to counteract the wickedness of mankind: “Everything they thought or imagined was consistently and totally evil” (Genesis 6:5). The fact that God did not consider His creation a mistake is proved by the world’s continued existence. We’re still here, sinful though we are. Praise the Lord for His grace: “Where sin increased, grace increased all the more” (Romans 5:20b), and “Noah found grace in the eyes of the LORD” (Genesis 6:8).”

I add:
We can surmise that God decreed Judgement upon an entire generation at the time, yet this is symbolic for the times to come, and as it the consistent theme in the gospels, to serve as a warning for the coming generations, especially in our times for us who have received the fullness of Revelation.