A huge thing, throughout the seven books and the films, Dumbledore expressed right at the beginning of the series - at the end of the Philosopher’s Stone, when Harry says ‘He’s not gone’, and Dumbledore says ‘No, but if the next person fights him, and the next person fights him, you know, we can keep him down’.
And that goes to the heart, really, of the series, which is the question ‘Why fight?’ Why fight?’.
We accept the inevitability of evil in the world, we accept that things can’t always be fair, we accept that things will never be perfect… Why fight? Why fight? And that’s the question that all of the characters answer in their different ways. And some characters say ‘I’m not fighting. I have to accept the inevitable’, and others characters say ‘I will fight ‘til I die’, to make the world a better place, to save a friend, you know, and I suppose that’s what the final battle is about. (x)