I used to believe that the destruction of the nations by Israel was God destroying a people for being so evil.  I was a little uncomfortable with that, but our creator has a right to do as He pleases.  I have since come to believe it is more complicated than that.  It is a purifying of the human race.

Genesis 6:4 states, "The Nephilim were on the earth in those days, and also afterward, when the sons of God came in to the daughters of men, and they bore children to them. Those were the mighty men who were of old, men of renown."  These men were giants and were Satan trying to corrupt the human race to prevent Jesus from coming to Earth as savior.  The nations in Canaan were also full of giants.  I believe these giants were also nephilim (demon/human hybrids) and that is why God had the Israelites destroy them, man, woman, and child.

The global flood, as well, wasn't just about destroying the people whose "every intent of the thoughts of his heart was only evil continually.," but was also to destroy the corruption of the human race.

This also matches many of the ancient myths and religions that talk about "gods" coming down and fathering children with women.  Since every person on Earth descended from Noah, they would all descend from people who had knowledge of these events.