It’s interesting that in Book One, chapters VI and VII it’s Boso who gets to critique one of the more widespread theories of the Atonement at the time of Anselm’s writing, the so-called Ransom theory favored by several of the Fathers.

In a nutshell, the Ransom theory teaches that, by sinning, humankind had put iself under the dominion of the devil and that Satan had acquired lordship over us. However, despite exercises this authority over humankind, Satan overstepped his bounds in killing Christ, because Satan had no rights over Christ since the latter hadn’t sinned. Thus, in illegitimately killing him, Satan forfeits his rights over the rest of us. I think it was Augustine who compared Christ to the bait on a fishhook: Satan snaps up the human being Jesus, but the divinity concealed within proves to be his undoing. A version of this theory seems to be at work in the depiction of Aslan’s death in C.S. Lewis’s The Lion, the Witch, and the Wardrobe. In betraying his siblings, Edmund has effectively made his life forfeit to the White Witch. But Aslan, agreeing to be slain in Edmund’s place deceives the Witch, who is unaware of the “deeper magic” and the fact that death will ultimately be unable to hold Aslan.

What Anselm/Boso takes issue with is the idea that the devil has rights over humankind such that God couldn’t release us from bondage to Satan merely by fiat:

I do not see the force of that argument, which we are wont to make use of, that God, in order to save men, was bound, as it were, to try a contest with the devil in justice, before he did in strength, so that, when the devil should put to death that being in whom there was nothing worthy of death, and who was God, he should justly lose his power over sinners; and that, if it were not so, God would have used undue force against the devil, since the devil had a rightful ownership of man, for the devil had not seized man with violence, but man had freely surrendered to him. (Book One, Ch VII)

Anselm/Boso doesn’t deny that Satan has a certain de facto power over humanity. After the fall we are certainly in Satan’s thrall and subject to his torments. What is denied, however, is that Satan has a de jure authority over us. The devil tempted humanity by means of treachery, so he can’t have acquired legitimate authority over us.

It’s allowed that God may justly permit the devil to be the agent of our punishment, but it doesn’t follow that Satan acts justly. “For man merited punishment, and there was no more suitable way for him to be punished than by that being to whom he had given his consent to sin. But the infliction of punishment was nothing meritorious in the devil; on the other hand, he was even more unrighteous in this, because he was not led to it by a love of justice, but urged on by a malicious impulse.” This is similar to the way in which, in the OT, God permits Israel to undergo certain hardships as a means of chastisement, without the agents of that chastisement (typically other nations) being just in themselves. God can use their malicious intentions as the agents of his justice. But they in no way have the right to do what they’re doing, just as Satan has no rights over us.

Thus, as a matter of justice, God is in no way obliged to respect Satan’s supposed rights. Therefore the Ransom theory, while getting at part of the truth – that, in Christ, God frees us from the power of the devil – can’t be the whole story.