system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: The plot of the Dark Knight.

As many times as I've seen this movie, I have never been able to full understand the film. And also, I would like an explanation of Batman's ending words of the film.
gpt
It helps to understand that the primary conflict in the film takes place not between The Joker and Batman, but rather between The Joker and Harvey Dent.

On the one side, you've got Harvey, who is determined to drag Gotham, kicking and screaming, away from the criminal slum it was turning into. Aided by Lieutenant/Commissioner Gordon, the Mayor and Batman, he's had a lot of success in that effort, and as such has managed to win Gotham's respect, trust and most importantly, hope.

Enter The Joker. He's a self-described agent of chaos. His goal is to create anarchy in Gotham, anarchy that could never really happen as long as Harvey Dent existed as a source of hope. So in order to create the anarchy he wanted, he had to beat Harvey Dent. But more importantly, he had to completely destroy Harvey's entire image, turn him from a symbol of hope to a symbol of despair.

Spoiler alert!

By killing Rachel the way he did--forcing Harvey and Rachel to confront her imminent death together--The Joker was able to create the despair in Harvey that he needed. The idea was that the people of Gotham would see that Harvey Dent, their White Knight, had turned into a psychopathic killer at the Joker's hand. All hope would be gone, and Gotham would descend into anarchy. Batman recognized that strategy and short-circuited it.