Jesus died for the sins of the world, as everyone died or is going to die except anyone other than Jesus died/s for their own sin. Jesus' death is exception in that, going by the law, He should have never died.
I hesitate to agree to the simplicity of the answers you've suggested.
Jesus died for the sins of the world, as everyone died or is going to die except that anyone other than Jesus died/s because of their own sin. Jesus' death is an exception in that, if going by the law, He should have never died. Is God just in that He required Jesus to die? I suppose, somehow, He must be even if it doesn't seem so at the surface and any quick answer would seem presumptuously premature.
Jesus is a remarkable being. Words fail in human language to do Him justice. But His death is about the assurance that sins were laid to His account. As you have alluded, He Himself committed no sin. So in order for Him to die, sin had to be imputed to Him.
His death isn't a matter of justice, but the greatest demonstration of love that will ever be done. I love the lines from many songs which speak to this:
Amazing love, how can it be
That Thou, my God hast died for me?
See from His head, His hands, His feet,
Sorrow and love flow mingled down.
Did ere' such love and sorrow meet,
Or thorns compose so rich a crown?
God was never under any obligation to save anyone who has sinned. But, in choosing to do so, He had to do so in accordance with Who He is.
The particular aspect of His justice demanded payment for sin...the soul that sinneth shall surely die. Subsequent revelation has revealed this payment to be an eternity in a place of torment.
It is just of God to allow a substitute for any sinner as long as the substitute paid all that was owed. Since only an eternal being can pay an eternal punishment, the substitute by necessity would need to be God Himself. And that's the very promise God made to Abraham...God will provide Himself a lamb...Gen 22:8.
I never intended to write so much, but I believe when God acts, He does so in accordance with who He is. So when one says that God is just and that He paid for everyone's sin I don't see any inconsistency. But when I understand that there people who are paying for sins that have already been paid for, I do not understand how that is just. And since I know that God is just, I find it inconsistent for Him to act in this manner.
I'm far from knowing everything...particularly when it comes to God. My own personal sense of mercy far outweighs my desire for justice, and I desire all men to be saved. But I am not encumbered with the immutable attribute of justice. And it is far easier to ignore the issue of justice than to try to explain it. But personally, I don't find compatibility between justice and requiring payment for something already paid for. If people do then that will be reflected in what they believe.