God is just, and the world He created is just, although it does not appear to be just at every moment. However, eventually God "will repay every man for what he has done" (Rm 2:6).
God is merciful. He treats us much better than we deserve. He is more than fair, better than just. God is not limited to being just, for His "mercy triumphs over judgment" (Jas 2:13).
Is God's being better than just unjust? Is it contradictory to be both just and merciful? The question in life is not so much: "Why do bad things happen to good people?", but "Why do good things happen to bad people?" God's mercy is more mysterious and therefore more problematic than His justice.
The Lord can be merciful without ignoring justice because He is "our Justice" (1 Cor 1:30). By dying on the cross in our place, Jesus fulfilled "the just demands of the law" (Rm 8:4). By fulfilling justice, He is free to move beyond justice — to mercy. Therefore, when Jesus forgave the paralytic's sin and claimed the authority to forgive sins, He was claiming to be God, our Justice, and even crucified Mercy.