I do not understand how an unconditionally loving Creator God could strike down all living creatures.Without trying to erase the controversy, I admit that scripture challenges us in many ways. Approaching scripture as a scientific text instead of what it is, a faith document, sharpens the controversy within and even prevents human hearts from conversion.
Conversion appreciates divine mercy. In the ancient Mediterranean world, a flood story circulated. Only the Genesis version offers a moral reason for it: human wickedness (see Genesis 6.5). With the flood, Genesis brings to an end the first human epoch of Adam and Eve. After the flood, God vows never again to destroy humanity. God did not require anything of Noah and his family in order for God to do that.
God, working through nature, sealed that covenant with the rainbow. In the ancient Near East, the rainbow was seen as a divine weapon. Genesis transforms the rainbow entirely. We will never understand God's ways. We can, however, be sure of God's mercy, revealed in so many ways and revealed fully in Messiah Jesus. To focus on God's destruction of the world and nearly every living thing misses God's mercy.