Quoting a couple of additional lines:
Him who disobeys,
Me disobeys, breaks union, and that day,
Cast out from God and blessed vision, falls
Into utter darkness, deep ingulfed, his place
Ordained without redemption, without end.
God knew hell might be needed, but this still does not say it was pre-created. (Of course if you read Milton’s God as having created a creature (Satan) he knew would sin, then the theological difficulty the OP is asking about is embodied right there–what difference does it make when hell was created if God knows in advance he will need to create it?)
The second bolded bit relates to what ralph asked–can Satan change his mind? Milton is contradictory on this point, IMHO because, despite several statements like the bolded part, he also has the following in Book IV (Satan speaking):
Me miserable! which way shall I fly
Infinite wrath, and infinite despair?
Which way I fly is Hell; myself am Hell;
And, in the lowest deep, a lower deep
Still threatening to devour me opens wide,
To which the Hell I suffer seems a Heaven.
O, then, at last relent: Is there no place
Left for repentance, none for pardon left?
None left but by submission; and that word
Disdain forbids me, and my dread of shame
Among the Spirits beneath, whom I seduced
With other promises and other vaunts
Than to submit, boasting I could subdue
The Omnipotent. Ay me! they little know
How dearly I abide that boast so vain,
Under what torments inwardly I groan,
While they adore me on the throne of Hell.
With diadem and scepter high advanced,
The lower still I fall, only supreme
In misery: Such joy ambition finds.
But say I could repent, and could obtain,
By act of grace, my former state; how soon
Would highth recall high thoughts, how soon unsay
What feigned submission swore? Ease would recant
Vows made in pain, as violent and void.
For never can true reconcilement grow,
Where wounds of deadly hate have pierced so deep:
Which would but lead me to a worse relapse
And heavier fall: so should I purchase dear
Short intermission bought with double smart.
This knows my Punisher; therefore as far
From granting he, as I from begging, peace;
All hope excluded thus, behold, in stead
Mankind created, and for him this world.
So farewell, hope; and with hope farewell, fear;
Farewell, remorse! all good to me is lost;
Evil, be thou my good; by thee at least
Divided empire with Heaven’s King I hold,
By thee, and more than half perhaps will reign;
As Man ere long, and this new world, shall know.
This stuff is reaaly wonderful reading. It also leaves it unclear whether or not Satan could change his mind.