It is sometimes stated that permitting a criminal to go unpunished is wrong, and is actually worse than the crime the criminal did.
a) It is worse when done by a judge.
b) It is not worse when done by a victim.
We can see this by considering that it is mercy not to drag everyone who has wronged us before a judge. On the other hand, it is injustice for a judge to fail to apply the law to the guilty.
We can also see this in the work of God. Christ shows us mercy. The Father shows Christ justice. Thus, God as victim shows mercy, while God as judge shows justice.
Furthermore, God as king shows mercy by permitting a substitute, but that mercy in no way diminishes justice, because he still demands satisfaction.