Justice requires accountability for wrongs committed. When we break God's moral law, justice requires that we be held accountable.
Jesus suffered for us to take the punishment that our sins require. Jesus took the penalty for us. That allows God to be a merciful judge, yet still fulfill all justice.
All hogwash of course, but just shows you how poorly thought out it all is.
A man is convicted of a crime and the penalty is $1 million. Another man steps up and says he will pay the penalty.
In a way, you are right about it not being just. The convict deserves the punishment, but gets mercy instead. This is the nature of mercy and justice. They are opposed. Jesus paying for our penalty intertwines the two.
You obviously don't believe any of this is true or right. I don't make it my aim to convince you or anyone else. I'd be kidding myself if I thought I could do that. Just presenting Christian doctrine as accurately as I can.
It's true that I don't believe it all of this, but let's assume for a moment that the following is true (quoting your previous post):
> Justice requires accountability for wrongs committed. When we break God's moral law, justice requires that we be held accountable.
To me another man suddenly showing up paying the penalty is completely contradicting and undermining this accountability.
When making the rules, a god could just say "I forbid these things, and there will be consequences for breaking the rules, but in the end I will forgive, because eternity is a long time and I'm so kind and loving". Instead, the Christian rules demand that somebody (who may or may not be the one who violated the rules) is hurt. So the Christian god orchestrated the killing of either himself or his son (or maybe a bit of both? So it's suicide and/or filicide. The holy trinity is confusing), because he kind of wants to forgive people, but the rules (that he himself made, and presumably could change, being all-powerful) demand that someone is hurt. Oh, and most interpretations hold this god to be omniscient, so in that case it's not like he didn't see this mess coming when he made the rules in the first place.
If you are completely in control of a situation and voluntarily set it up in such a way that in the future you have to hurt yourself (or have you children hurt themselves) to get things to work out, is it really self-sacrifice? Having Jesus die for our sins makes the Christian god seems like a heroic firefighter who is also the arsonist.
Firstly, if God is the one who exercises judgement in the first place in the form of who goes to heaven and who does not, then (as is written elsewhere in the Bible) what God says is good, is good, and what God says is bad, is bad, for no intrinsic reason other than God says it is so. When I do the good, I want to do it because I have some stock in the moral value or principle being maximized, not because some deity arbitrarily commanded it.
Secondly, if God is the source of all moral judgement, then he could just arbitrarily decree that repentance is now a thing instead of making Jesus suffer for our sins. In the same breath Christians assert that moral values come from God (if God says it is good, it is good, if God says it is bad, it is bad), but that Jesus had to suffer, to what, to make God decide to be more lenient with how he interprets our sins? How is that supposed to be inspiring? Makes God seem sadistic more than anything. But then when God contradicts his own moral values (i.e. murder in some of the old parables) Christians of course once again say no, this isn't immoral because God says it is not. Which is it? Either morality is whatever God says it is, and Jesus dying on the cross served absolutely no purpose, or God is a sinner and cannot/should not contradict his own moral tenants (but does because there is no one there to "judge" him), in which case Jesus dying on the cross makes some sense because morality is some force above and more powerful than god.