Now the earth was corrupt in God's sight and was full of violence. God saw how corrupt the earth had become, for all the people on earth had corrupted their ways. 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. I am surely going to destroy both them and the earth. So make yourself an ark of cypress wood; make rooms in it and coat it with pitch inside and out. This is how you are to build it: The ark is to be 450 feet long, 75 feet wide and 45 feet high. Make a roof for it and finish the ark to within 18 inches of the top. Put a door in the side of the ark and make lower, middle and upper decks. I am going to bring floodwaters on the earth to destroy all life under the heavens, every creature that has the breath of life in it. Everything on earth will perish. But I will establish my covenant with you, and you will enter the ark - you and your sons and your wife and your sons' wives with you. You are to bring into the ark two of all living creatures, male and female, to keep them alive with you. Two of every kind of bird, of every kind of animal and of every kind of creature that moves along the ground will come to you to be kept alive. You are to take every kind of food that is to be eaten and store it away as food for you and for them."
As with other cases of mass judgment in the Old Testament, the following points apply:
The people judged were guilty of very grave offenses
The people judged had the opportunity to repent
Righteous people were spared judgment
1. The world before the Flood was completely evil:
The Lord saw how great man's wickedness on the earth had become, and that every inclination of the thoughts of his heart was only evil all the time. The Lord was grieved that he had made man on the earth, and his heart was filled with pain...Now the earth was corrupt in God's sight and was full of violence. God saw how corrupt the earth had become, for all the people on earth had corrupted their ways. (Gen 6:5-6, 11-12)
If the world was full of violence, murder was presumably common, and quite possibly other forms of violence (beating, maiming, torture, etc.) were common as well. The population was being judged not for minor offenses but for the most serious of crimes.
2. The population at the time was not far removed removed from Adam and Eve. Their experiences had been handed down to the current generation, for Noah's father knew the story of the Fall (Gen 5:28-29); Cain's experiences were also handed down for several generations (Gen 4:17-24). Furthermore, Noah was a "preacher of righteousness" (2 Pet 2:5). The people around him would have seen his example and most likely would have noticed that he was building a large boat and stocking it with supplies; even if Noah hadn't warned them about the judgment, this would have been enough to provoke curious questions. If they had changed their ways even at that late point, they would have been spared (Jer 18:7-
3. Not everyone died; Noah was spared because he was righteous, and God graciously extended mercy to his extended family, even though they were not necessarily as righteous. It's worth noting that Noah's father and grandfather died shortly before the Flood (Gen 5:25-31, 7:6); possibly God delayed judgment so that the righteous people of older generations would die naturally of old age (cf. 1 Pet 3:20). However, innocence is not the same as righteousness; people (e.g. children) who had not committed either good or bad actions were morally neutral, as opposed to people who had consciously chosen good over evil. (See What about the children? in the article on genocide.)