God tells us why He destroyed the earth with a flood in the time of Noah in Genesis 6:5–7. "The LORD saw that the wickedness of man was great in the earth, and that every intention of the thoughts of his heart was only evil continually. And the LORD regretted that he had made man on the earth, and it grieved him to his heart. So the LORD said, 'I will blot out man whom I have created from the face of the land, man and animals and creeping things and birds of the heavens, for I am sorry that I have made them.'"
What was the reason for the flood in the time of Noah?
We know that the world was absolutely corrupt, and though verse 6 above says God "regretted" making humans, the direct translation of the Hebrew nahem is "grieved." God grieved the state of affairs. He desired, and always has desired, a right relationship with people. The people at that time were so evil, so in rebellion against God and His decrees, that God was lamented, then decided to take radical action.
Remember, God is loving and merciful, but He is also holy and just. People continually sinned against Him, so He responded with resolve to act justly, but also salvage the race of humans. Genesis 6:8 tells us that Noah found favor in God's eyes; verse 9 describes him as "a righteous man, blameless in his generation. Noah walked with God." God chose Noah and his family to be saved, but assigned them the task of spending 120 years telling others about the coming destruction while preparing the ark.
Additionally, Genesis 6:2 tells us that "the sons of God saw that the daughters of man were attractive. And they took as their wives any they chose." This displeased God as well. In Hebrew, these "sons of god" are "bene elohim," a term most often used to describe angels (Job 1:6). Genesis 6:4 tells us that the offspring of these beings with human women were Nephilim. This word literally means "fall" but is often translated "giants." Some theologians say one strong reason for the flood is that God would not allow these creatures to continue to live. All of them, as well as the descendants of Cain, were wiped out in the flood.
All people today are descendants of Noah and his family. Noah was a descendant of Seth. Noah is listed in the genealogy of Jesus Christ, the vital and sole conduit for people to come into a right relationship with God.
Questions about Noah's ark: How long did it take Noah to build the ark? How long was Noah on the ark?
Was the Noahic flood global?
Nephilim - Who or what were they?
What is the Epic of Gilgamesh? What relation does it have with the biblical Flood?
How important would the discovery of Noah's Ark be?
Truth about Creation