J.K. Rowling did an interview recently with a Dutch newspaper, and it included (among a lot of other things) her thoughts on destiny and free will. (For those who care about spoilers, you might not want to look at the interview or read the rest of this post.)
I have to confess that I'm a little disappointed in her response. She's very smart and well-informed about intellectual matters. But I have to wonder if she presents a false dilemma on this issue, and I'm not even sure the view she expresses here fits well with the books she wrote.
Your books are about the battle between good and evil. Harry is good. But is Voldemort pure Evil? He is also a victim.
He is a victim, indeed. He is a victim, and he has made choices. He was conceived by force and under the influence of a silly infatuation, While Harry was conceived in love; I think the conditions under which you were born form an important fundament of your existence. But Voldemort chose evil. I've been trying to point that out in the books; I gave him choices.
So far so good. It's important to distinguish between being forced into good or evil because of what happens to be true about your conception and making choices. This still doesn't say anything about the metaphysical status of free will. A libertarian will hold that these choices can't be caused by prior events if they're to be free, and a compatibilist will allow that they might be caused by prior events while still being free, because the distinction here is between being forced into something no matter what your own choices would be (merely because of the circumstances of your conception) and making choices (which doesn't yet say anything about whether those choices have explanations and if so what the explanations are).
But where she goes from here is what I find problematic: