I think it would be helpful if you focused your question more specifically.
God’s mercy is a sign of grace (God’s love poured out for humanity), and this mercy oftentimes manifests itself in the compassionate treatment of a wrongdoer. That doesn’t mean that punishments are not given, but take Cain for example.
Cain killed his brother Abel, but God did not kill Cain in response. Instead, God marked Cain and made him a ceaseless wanderer of the earth.
Concerning the flood, God was grieved by the wickedness of humanity and sought to start over. Ironically, God forms a covenant after the flood dries up and promises never to do something like that again (by means of a flood). Even the rainbow is set in the sky as a sign of God’s grace and mercy – and a reminder of the promise.
In the chapter preceding Sodom and Gomorrah (Genesis 18), Abraham and God have an interesting exchange as Abraham pleads for God to spare innocent people. If you follow the formula of the dialogue, then it is fair to conclude that God would spare the WHOLE town for the sake of even one innocent person. That is mercy. Unfortunately, we know how that story ends … so no innocent people …