Mercy is God not giving people what they deserve. You, like Israel, stand before God condemned because of your sins. You have made your own false gods. You haven’t given God the glory for the gifts and deliverances He has given you. You have used His gifts for sin. You have used your gift of speech to lie, be vulgar, and not give Him His due glory. You have used your sight to distract your mind from worshipping Him and looked at evil things. You have used your health to create idols of activities that draw your heart away from Him and you have used your health to pursue sexual sin. You have used the wealth that He has given to purchase things that lead you into sin. 

The justice of God is that He fairly struck down Israel. But we need His mercy, not His fairness. It is not fair that He would offer us forgiveness. The point of the law is to show us our flaw and our need for the Savior. 

The law brings death, but Jesus fulfills the law and brings life. I love this picture from the foot of Mount Sinai. When the law comes down, 3000 people die. This is 50 days after the first Passover. On the Passover, when Jesus gave Himself as a ransom for many, 50 days later, when the Spirit fell, 3,000 people were given forgiveness and life. 

If you want the justice of God, trust in your ability to fulfill the law. If you want the mercy and forgiveness of God, trust Jesus.