Thanks for your question.
All you have to do to find out why the world was destroyed by a flood in the days of Noah is to read Genesis 6:6,7,11-13.
"6, 7 And the LORD was sorry 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.”
11-13 Now the earth was corrupt in God’s sight, and the earth was filled with violence. And God saw the earth, and behold, it was corrupt, for all flesh had corrupted their way on the earth. And God said to Noah, "I have determined to make an end of all flesh, for the earth is filled with violence through them. Behold, I will destroy them with the earth."
Compare this with what Jesus said: Matthew 24:37 "As it was in the days of Noah, so it will be at the coming of the Son of Man." The Lord also compared the day of his return to that of the destruction of Sodom in Luke 17:29,30 "But the day Lot left Sodom, fire and sulphur rained down from heaven and destroyed them all. "It will be just like this on the day the Son of Man is revealed."
God has not left the world to itself. He has appointed a day in which He will judge the world in righteousness through the man whom He has ordained, Jesus Christ. It is clear that the violence, corruption, and sexual immorality of our day is a fulfillment of God's word through His son. Because of that, we eagerly expect the return of the Lord Jesus very soon.
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