Milton's God: Freedom and Necessity

Matthias Grünewald, Crucifixion

Isenheim Altarpiece

(As Mounted on Wikipedia)

On the Milton Listserve, the scholar John Rumrich has noted that Milton's God acts freely in all that he does:

[It] is certainly correct to say that the Son plays no necessary part. Milton unequivocally asserts in Christian Doctrine 1.5 that even the Son's generation occurs freely, according to the Father's sovereign freedom. In PL the character of God insists that necessity keeps its distance from him, and the Son voluntarily chooses to do what he will do. The method of salvation they light upon is not the necessary one. It's the one they extemporize, so to speak. I don't see the problem with acknowledging this. Milton never associates the necessary with the praiseworthy so far as I can remember.

This seems correct to me, but I've raised a question on the Milton List that I'd like to reprise here. As John Rumrich has noted, Milton's Father-God is not bound by necessity but acts freely in all things, including in his offer of grace. However ... there may be some things "required by the theology of the poem," e.g., for free grace to be offered, a sacrifice is necessary to preserve divine justice, as PL 3.203-216 informs us:

But yet all is not don; Man disobeying,
Disloyal breaks his fealtie, and sinns
Against the high Supremacie of Heav'n, [205]
Affecting God-head, and so loosing all,
To expiate his Treason hath naught left,
But to destruction sacred and devote,
He with his whole posteritie must dye,
Dye hee or Justice must; unless for him [210]
Som other able, and as willing, pay
The rigid satisfaction, death for death.
Say Heav'nly Powers, where shall we find such love,
Which of ye will be mortal to redeem
Mans mortal crime, and just th' unjust to save, [215]
Dwels in all Heaven charitie so deare?

Why is this sacrifice (i.e., the "death for death") necessary for justice to be preserved? Because the Father-God had previously commanded that one eating from the Tree of Knowledge of Good and Evil must die? -- presumably because, as stated above, one who commits treason through disloyalty to God must die?

But why, in the Miltonic economy of salvation, would God be required by his divine justice to demand a sacrificial death in order to offer his saving grace?

In short, why does Milton think that somebody must die?