Why did God say He would destroy all flesh with the earth in Genesis 6:13?
God declared He would destroy all flesh with the earth because the earth was corrupt before Him and filled with violence. All flesh had corrupted its way upon the earth, so the end of all flesh had come before God. This corruption and violence led directly to the Noahic flood judgment.
Les explains that Genesis 6:11-12 describes the earth as corrupt before God and filled with violence, and all flesh had corrupted its way upon the earth. In verse 13 God says the end of all flesh is come before Him because the earth is filled with violence, so He will destroy them with the earth. Les ties this directly to what led up to the Noah flood. He also compares it to the approaching same kind of judgment, when God will pour out His wrath and vexation on Christ-rejecting man during the final seven years of tribulation. He connects this to Romans chapter 3, where Paul lists the deeds of the ungodly and says the whole human race outside salvation has gone out of the way, become unprofitable to God, and none does good.
And God said unto Noah, The end of all flesh is come before me; for the earth is filled with violence through them; and, behold, I will destroy them with the earth.