Divine Justice: Free Choice or Necessity?

Did God have to punish his Son for our sin, or is this simply the way that he chose to save us?  In other words, could God have made satisfaction for sin in another way?  This question is often posed as a difficult one, but really it ought not to be.  To suppose that God poured out his wrath on his Son when really it wasn't necessary makes this demonstration of God's justice merely an act of his will and not of his nature.  And if it was merely an act of his will and not of his nature, it follows that this willingness to punish Jesus in our place is rather an instance of injustice, if in fact God could have done otherwise.