Well, this theme should come as no surprise, given that Harry Potter and the Deathly Hallows, Part 2 features the epic showdown between Harry (a super brave, insanely nice, and more or less innocent teenager) and Voldemort (a psycho murderer who literally would do anything and kill anyone to get what he wants).
Of course, Harry and Voldemort also share some similarities—you know, like the ability to talk to snakes, their upbringing as orphans, and their affection for Hogwarts—so you can't quite say they are total opposites…but they're definitely the faces of two very different ways of life.
Questions About Good vs. Evil
Why give Harry and Voldemort some common characteristics? What does that say about how we think about who is "good" and who is "evil"?
Do any characters live between the whole "good" and "evil" divide?
Is Snape redeemed for his past "Death Eater" sympathies in the end?
Is Dumbledore as pristine/good as we originally thought?
Chew on This
Harry and Voldemort's similarities suggest that there are really no such things as pure "good" and "evil," per se—it's all about how you're raised and what happens to you. If Voldemort had had more love in his life, perhaps he wouldn't have gone so dark.
Harry lived without love for a long time (remember the Dursleys?) and still ended up being a great guy. Some people are just stronger, braver, and better than others.