Justice in the sense that sin and lawlessness will be punished. New “creation” in the sense that God destroyed the corrupted flesh that was on the earth and started again with Noah and his family. Mercy in the sense that God had to put an end to the violence and corruption on the earth before mankind wound up destroying themselves. Covenant relationship in the sense that God started with promises made to Noah and his family (will never destroy the earth again in a flood).
What seems missed in the questions about God’s justification for sending the flood is the demand for justice upon evil, corruption and violence. What is missed is the depravity of what existed during the pre-flood days. People do not want to believe that people can be that depraved and sinful. How much “blood that cries from the ground” needed to occur before a punishment? The “blood crying” is a symbol of the soul crying for its right to live (Lange).