There is nothing unjust about an innocent person choosing to die for the guilty. But would a just judge accept the death of the innocent in place of the guilty?
What would you say of an earthly ruler or king who said that he would put an innocent volunteer to death in the place of a vile murderer?
Do you think the king's subjects would praise him for doing so?
Yes, Jesus died for us. But did the Father pour out his wrath on him? Did God treat him as though he were guilty? Or did God allow a miscarriage of justice so that we might trust Jesus?
It is important to note that none of the early Church fathers said that the Father poured out his wrath on his Son. And there is not a single verse in the Bible that says the Father poured out his wrath on his Son (that idea has been read into the text). If you’d like to know how the idea that the Father poured out his wrath on Jesus became dominant, read Christus Victor by Gustaf Aulén. (Aulén is mistaken on some issues, but he is right about how the doctrine of substitutionary atonement was introduced into the Church. For an in-depth look at the problems with penal substitutionary atonement and a view which has strong biblical support, see Atonement, Justice, and Peace: The Message of the Cross and the Mission of the Church by Darrin W. Snyder Belousek.)
"Strange that in a Christian land it should need to be said, that to punish the innocent and let the guilty go free is unjust!" - George MacDonald