Free will. Some have called it the greatest gift bestowed on humanity. […] Others say free will is a crap myth. (Prologue.1-3)
These are the first lines of the book, which lead you to believe that fate and free will will be a more important theme than it actually turns out to be. Although there's a lot of talk about it early on—and maybe it will come back in a future book—it's a fairly minor theme in this installment.
I, personally, don't believe in this crap. I say I control my destiny and my life. (Prologue.9)
Nick is required to say this, because a protagonist who thinks that has no control over his life would be pretty hopeless to read about. But what's the reality? How much control does Nick really have, if you think about it?
I am what I have become because of the interference and secrets of one creature. Had things been done differently, my life would have been a whole other enchilada. I would not be where I am today and I would have had a life worth living instead of the nightmare my life has become. (Prologue.12)
Does Nick not realize that he is contradicting what he just said about controlling his own destiny and life? It's possible the person talking here is actually future Nick, who goes by the alias Ambrose. By going back into the past, Nick wants to see if he can change his own destiny. (But then wouldn't the future be different, anyway? So how could future you go back in the past to fix things if things have already been fixed, and future you is already just fine? Don't ask us.)
His mom had already road-mapped his entire future with no diversions or pit stops allowed. (1.12)
As a teenager, it can be especially difficult to believe that you have any sort of free will, since your parents control everything you do. And when you're parents aren't controlling you, then there's school. And then at school, there is the terrifying, Darwinian nature of social life that controls you, too. Yikes.
You were human once too with no knowledge of your true birth or destiny. (7.156)
This is Acheron thinking to himself. That whole paragraph from the prologue about the freight train was Acheron's theory of free will, as explained to Nick. So it makes sense that Acheron feels he has a destiny. He feels his own life is mostly out of his control.
What irony, really. The fate of the entire world, of humanity and demonkyn, was in the hands of a fourteen-year-old boy who had no idea of the untapped powers he'd been born with. (11.43)
Nick may not believe in fate, but other people—and demons—sure do. And they think that their only hope is Nick. That is sure to change Nick's life in ways that are somewhat out of his control.
But he had no choice. When he was summoned, he had to obey. Those were the rules and any hesitation would only end badly for him. (11.5)
Caleb is also a victim of fate. He is being controlled by Nick's father. One of the only ways he can rebel against his harsh master is to try and change Nick's fate. So it turns out he has some free will, after all, right?
Nick Gautier had no idea how the random small decisions he was making right now would turn him into the beast Ambrose had become. (13.175)
Perhaps the Nick in the prologue is actually Ambrose. He got to be Ambrose by accepting his fate and letting it carry him down a dark path. But now, Ambrose wants to change his own fate by encouraging Nick to make different decisions.
"Unfortunately, Nick, you are living my life and God help us both, we're making all new mistakes." (18.217)
Maybe free will takes the form of mistakes, because these "mistakes" could change Nick's fate. Maybe we have a certain path or destiny, but we're still presented with multiple choices along the way that gives us chances to exercise some free will. It might be limited free will, but it's still free will.
Destiny is a freight train rolling along on a set course that only the conductor knows. When we get to the railroad crossing in our car, we can choose to stop and wait for the train to pass us by, or try to pull out in front of it and beat that bad boy across. That choice is our free will. […] If it is our destiny to be hit by the train, we will be hit by the train. The only thing we can change is how the train turns us into hamburger. (Prologue.4-7)
This is a good explanation of the complicated relationship between fate and free will. Is it really free will if your fate is still predetermined? Is there some kind of middle ground between fate and free will? Maybe we have a little of both?