Can you imagine a judge who consistently let those who everyone (including and foremost the Judge Himself) knew was guilty go free? What would be the outcry? Whether it was in a small town or on the federal level, you can hear the clamor rising to jet-engine decibels. There would be protests and petitions and all sorts of things to make sure he was ousted.
Now run that up to a cosmic scale.
According to Romans 3, God "passed over" sins previously committed. David's adultery and murder. Moses' murder. Abraham's deceit. Solomon's idolatry. Nehemiah's doubt. Job's questioning.
Now if God had passed over those sins, how could He be just? Because He was planning on punishing all sin at the cross. That's the place where He could be both just and the justifier of the ones who have faith in Jesus.
Their sin. Our sin. All sin. Done with, once for all, on a hill where stood a cross with a naked Jewish prophet who just so happened to be God in human form.