God destroyed the earth with a flood due to its excessive violence.  Violence destroys freedom and freedom is the ultimate form of good.  (The New Testament declares that “it was for freedom the Christ has set you free”).  Since evil is the absence or privation of something good, then the privation of ultimate good by means of violence required a dramatic response from the source and Judge of all that is Good. 

The fact that God let the earth and its inhabitants become so corrupt that only a few people and none of the land could be saved indicates His long-suffering grace to let people make a free will choice for ultimate Good in the form of God and His Ways.  God balances His Love and His Justice, His Mercy and His Wrath in ways that we humans cannot fathom.  The infinite and eternal nature of God holds measureless wisdom and power to rightly decide all matters of life and death.