If Christ is God, and He is, then He is the same in the revelation of both His justice and His mercy. God's mercy and His justice are never at odds. He shows abundant grace to us in order to give us time to repent. We should never take His mercy for granted, and we should never demand that He be merciful, claiming that He has been unjust when, in fact, He has been just. A holy God is just and merciful.
Mercy is free and given voluntarily.
If God were to deal with all of us according to justice, each of us would perish instantly. In the circumstances above, God chose to show His justice based on His holiness. Those of us who continue to live, even though we sin daily, live according to His mercy - something that is freely given by His grace. Thank God for the hope He has given in Christ Jesus our Lord.