The verdict lands, and it lands on Noah's ear first. Targum Pseudo-Jonathan on (Genesis 6:13) gives us the direct speech: "The end of all flesh cometh before Me, because the earth is filled with rapine by their evil works; and, behold, I will destroy them with the earth."
Again the Targumist returns to chamas — violence, robbery, exploitation. This is the definitive cause. The Talmud (Sanhedrin 108a) notes that the generation of the Flood was guilty of every sin, but the decree was sealed specifically on account of theft. When no one's property, body, or dignity is safe from anyone else, a civilization has already chosen to dissolve itself. God simply confirms what has already happened.