The Wisdom of Israel, by Edwin Collins, , at sacred-texts.com
The people who perished at the time of the flood, says Rabbi Yochananan, received their punishment, and have a share in the world to come. But why was the earth destroyed, and why did the animals perish? Rabbi Jodon said: "It is like a certain king who entrusted his son to a tutor, who led him forth to evil ways, to vice and crimes so vile that the king, in anger, slew his son. Then said the king: 'No one led my son into evil ways but this man. My son has perished, shall this one remain alive?'"
The world and its abundance led man to sin; the animals, even, were corrupt.
The whole human race are God's beloved children; for them all was created. The world had caused their ruin; it must perish with them.
Or, says Rabbi Pinchas, It may be likened to a king who caused his son to marry, and prepared a splendid bridal chamber for him. But the son turned to evil ways, and the king was angered and slew him. Then he entered the bridal chamber and broke down its walls and tore the curtains and destroyed all the ornaments. The king said: "Nothing of all this was made except for my son; now he has perished, shall these things remain?"