Confusion reigns in the "new Church" regarding the virtue of justice. Now, most Catholics understand "social justice", but few understand the justice of God, Who is All Just.
Here is one passage to start the examination of God as Just here.
2 Thessalonians 1:6-9
Douay-Rheims 1899 American Edition (DRA)
6 Seeing it is a just thing with God to repay tribulation to them that trouble you:
7 And to you who are troubled, rest with us when the Lord Jesus shall be revealed from heaven, with the angels of his power:
8 In a flame of fire, giving vengeance to them who know not God, and who obey not the gospel of our Lord Jesus Christ.
9 Who shall suffer eternal punishment in destruction, from the face of the Lord, and from the glory of his power:
We tend to understand mercy, which is defined as the giving of compassion when punishment would be due.
Mercy is not merely the overlooking of the error, but the admitting of an error and then the dispensing of punishment.
Mercy is not denial of sin, of evil.
Justice looks at "what is morally right and fair" and decides on punishment or mercy. Now, justice does not have to entail mercy, mercy is not a "given".
But, justice must exist in order for mercy to exist.
Without some type of judgement, mercy does not make sense.
Justice is one of the cardinal virtues and involves the intellect, reason and freedom. The last word forms a real obstacle for some Catholics.
Many Catholics deny free will. We all have the freedom to choose good or evil, a Godly life, or a life without God. Too many people want to deny culpability.
Freedom is one way in which we are created in the Image and Likeness of God. The other ways we are like God may be seen in our ability to think, our intellect, and in our creativity, our ability to create.
We are free because God has made us in His Image and Likeness and He is free.
Therefore, God represents to us pure Freedom, which leads to pure Mercy and pure Justice.