Always Bad in Noah's Time.
Why people weren't always bad.
Jesus said that people were merry-making and marrying in Noah's time--
Matthew 24:38 For in the days before the flood, people were eating and drinking, marrying and giving in marriage, up to the day Noah entered the ark; 39 and they knew nothing about what would happen until the flood came and took them all away.
Genesis says everyone was wicked in Noah's time, always doing bad things‑‑
Genesis 6:5 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.
Genesis 6:11 Now the earth was corrupt in God’s sight and was full of violence. 12 God saw how corrupt the earth had become, for all the people on earth had corrupted their ways. 13 So God said to Noah, “I am going to put an end to all people, for the earth is filled with violence because of them."
Jesus' words "the people were eating and drinking, marrying and giving in marriage" contrast with Genesis.
It isn't an evil world that eats, drinks, and gets married. Yet Genesis tells the readers--
"[E]very inclination of the thoughts of the human heart was only evil all the time."
Genesis doesn't describe the ways in which people had corrupted themselves.
Still, however, getting married isn't a corruption.
Jesus says "they knew nothing about what would happen until the flood came and took them all away."
So Noah didn't warn them about the coming flood.
Meantime, the Bible says that eating, drinking, and getting married--doing those things, they were doing "only evil all the time." (Um ...)