I’m not saying Rutherford is right or wrong, just noting particulars.
Taken from Guy Richards, “Samuel Rutherford’s Supralapsarianism Revealed.”
“Rutherford believes that God, although just, merciful, and good, is under no compulsion” to be just to his creatures ad extra (Richards 32). “But once he decrees to act ad extra, he is bound to do so.”
In other words, for the Reformed voluntarist tradition, paraphrasing William Twisse, the only thing that limits God’s free will (to act ad extra) is his decree. The Scotists aren’t saying that God’s will makes just anything right. Rather, they are saying, given what God has indeed ordained to be the case (potentia ordinata), God is bound to will x.
Classic example: Was Jesus’s atonement necessary? Rutherford has usually been understood as saying, “No. God could have forgiven sins otherwise.” But I don’t think this is exactly what he said. Rather, as Richards points out, it is contingently necessary (n 29). Since God has chosen to act this way towards creatures, and to punish sin this way, has it become necessary. But he was under no obligation to decide to act this way.