Buddha Dharma
Dharma Practitioner
God set up a system of laws. Law breaking must be punished or God would not be a just God.
Right, and as the OP stated- it isn't just to punish someone else for the crime. According to Christianity (typical orthodoxy): God has done so not once, but twice.
God has made Jesus take the sins of everyone allegedly, which I very much doubt. He has also, according to the usual theory of Christians: made all humankind take consequences for the sins of Adam.
Besides being unjust- this is in very contradiction to the Hebrew Bible, but I don't think that'll bother Christians very much. The Hebrew Bible is only useful in as far it agrees with the New Testament, right?
Christians could just admit they took sin and made it into something Jews never thought it was- with much more far-reaching, cosmological implications.
Sin has become a collective force under the Christian worldview, where under the Jewish one it was merely missing the mark. Sin has it's own reality within Christianity.