For the first time ever, Harry is totally at loose ends in Harry Potter and the Deathly Hallows – he's lost the guiding lights he had in earlier books (Dumbledore, Sirius). And with no one around to tell him what to do, he's got to make his own decisions. However, he isn't just any normal seventeen-year-old boy deciding ordinary things like which college to go to or which car to save up for. No, instead, Harry's decisions basically determine the fate of the world. And trust us, we're glad we're not in his shoes… the pressure to do the right thing is incredible! Harry's an adult for the first time here, and he's learned that he's fully responsible for all the choices he makes – and that these decisions will effect everyone else… in the world.
Of course, though, all of the characters – not just Harry – are absolutely defined by their choices throughout the series. And all of them are given a chance at redemption through these decisions – yes, even those who we may write off as evil. (Just think of Snape, Regulus Black, and even Draco Malfoy.) Even Voldemort, at the end, is given a chance at redemption, but which he refuses. The theme of Choice was conscious on Rowling's part. Here's what she has to say on the matter:
And it's about choice. And you are shown that Voldemort. I mean, it – I suppose we're going to call him a psychopath. But he's so, in many ways, he is what he is and he's beyond redemption. Although this being Harry Potter and because I can take liberties because I have magic in my world, it is shown at the very end of the book that he did have a chance for redemption because he had taken into his body this drop of hope or love [Harry's blood] […] So that meant that if he could have mastered the courage to repent, he would have been okay. But, of course, he wouldn't. And that's his choice. But the people around him, that's what's more interesting in a way. The people who were drawn to him for protection, for power, sadism. But people who do have a choice, did make a choice, like the Malfoys of this world. And I think that's always worth examining why people choose to make those decisions. (source)