Forgiveness of sins is not just. Quite the opposite. It is unjust because it is merciful. Christianity is not a belief in justice - it is a belief in mercy. We Catholics and Orthodox pray Oh, God, we pray that you have mercy on us, a sinner. We do not pray that God takes His Justice upon us. That would be a bad thing for us, and would sentence all of humanity in with the lord of this world in the end.
I'm sorry you don't like how that verse was worded in I John 1:9, but I'll quote it again for you:
If we confess our sins, he is faithful and just, to forgive us our sins, and to cleanse us from all iniquity.
Again with the torturing of words! ;o)
God showed us mercy for the very reason that he took the justice we deserved when Jesus died in our place. Christ TOOK our justice upon himself so that we might be made the "righteousness of God in Christ". That is mercy AND grace, my friend. We do not get what we deserve and do not deserve what we get.
Giving blanket forgiveness is no justice, for sure. What would we think of a judge who released repeat sex-offenders, molesters and murders because he "loves" them? certainly no "just!"
But the Christian God doesn't forgive unconditionally. First he demands that one believes he is a real God. Second we must believe that God took on flesh and suffered and died for our sins. Third, he demands that we be baptized in his name. Fourth, he demands confession and repentance in order to repeatedly look the other way in order to keep us "justified" in his eyes.
That's not unconditional mercy! It is conditional and biased injustice.
But you can't repeatedly commit the same sin, confess and "repent" of it, only to commit it again, and expect mercy ad infinitum.