My friend loaned me "The Bible for Dummies" and I found it very helpful! I've been reading the Bible for the first time recently and I am still in the Genesis. The Bible for Dummies explains everything in plain english and now I read it everytime I finish a chapter in the bible. This is taken from "Bible for Dummies":
Understanding why God would destroy his own creation is very important.
Shortly after the Flood (as you may have read), God gives Noah some good news and some bad news. First, the good news: "I will never again curse the Earth... nor will I destroy every living creature as I have done." (Genesis 8:21) Now the bad news: "for the inclination of the human heart is evil from its youth." In other words, the human condition hasn't changed as a result of the flood. Just as every inclination of the human heart was evil before the Flood, so it is after the Flood. So if evil still exist, why did God send the flood?
The purpose of the flood seems to be related to god's "good news" item. The Earth was "cursed" before the Flood. Why? Just as when Cain killed his brother Abel and the earth "received his blood,", the Bible says that after Cain's time, "violence increased on the earth," and, therefore, the earth took in a lot more blood.
By sending the flood, God not only judges humankind for its wrongdoings, He cleanses the soil of pollution. In short, God is being presented as the great Cosmic Ecologist, who cleans up the Earth from the damage caused by Sin. This motive for the Flood also explains the commandement, or mandate, God gives Noah after the flood.