Yesterday I finally finished this book, and I must say it left quite the impression.
Milton proposes in the very beginning to "justify the ways of God to man", an act which the classical biblical interpretation of God the book offers would probably condemn as presumtous and blasphemous; so I couldn't help but wonder throughout the book what his solution to the ever present theological problem of free will vs God's omniscience and trials would be, and in the end I found a potential answer.
Now since a lot of scholars with a much greater understanding than me have already dissected this book in many essays, I'll keep this brief.
I think Milton's implication was that man failing God's trial and choosing to pursue the knowledge of good and evil may actually be a good thing, and God's true plan, because only by abandoning their innocence and then finding it again can they truly be perfect.
In one of the final verses of book 12 Michael tells Adam as he is led out of Eden that humanity will one day "not be loath to leave this Paradise, but shalt possess a paradise within thee, happier far".
Not an equal paradise, not a physical heaven to ascend to one day, but an internal spiritual peace that will eclipse what they had lost.
This prediction is in contrast with Satan's condition, as throughout the book there are references to the "hell within" him, which renders him incapable of finding peace even once he reaches Eden, an heaven comparable to that he had lost, and leads him to evil time and time again.
While the humans were naive and innocent when they chose to betray God's command, Satan knew good and evil and chose the latter. His real crime, unlike that of man, wasn't doubt, nor was it a wish for equality, it was his envy of God's place and power.
In the end God's punishment of him reveals almost superfluous, because it couldn't possibly outweigh the doom he imposed on himself by following his lowest instincts, which he will truly never escape.
by DurkNya