Alan Moore and Brian Bolland gave us the best Joker story of all time in 1988. In a one-shot comic, they managed to tell a three-act story of Joker's life within 48 pages. Joker's origin-story (his first act in life) was compelling, but what made it the definitive origin-story was something Joker said later to Batman: "If I'm going to have a past, I prefer it to be multiple choice!". This one particular line demonstrates Joker's shattered psyche. It has been adopted by other storytellers in Batman's history; most notably, Paul Dini and Bruce Timm incorporated this multiplicity factor in "Mad Love", in which Joker confides in Dr. Harleen Quinzel and triggers her transformation into Harley Quinn... Batman later on tells Harley that it is one of the many different versions of Joker's past. We also see this element in Christopher Nolan's Dark Knight (2008); it manifests in Joker telling a different story every time he talks about the way he got those scars.
Going back to The Killing Joke: We see a plausible version of that "one bad day" that made Joker break. We know his past is multiple choice - maybe what we heard was the correct version, or maybe not... Regardless, he had "one bad day". Just like Bruce Wayne when his parents were shot in the Crime Alley after they left the movie theatre (showing The Mark of Zorro)... What I'm trying to say is that the book draws many parallels between Batman and Joker. They both had one bad day; the both were taken to the edge of the abyss, "the place where you don't care anymore; where all hope dies" (taken from Batman and Batgirls conversation earlier in the movie):
On that day, Batman was born, and Bruce Wayne died. Batman made his life mission to make sure that no other kid would go through what Bruce Wayne experienced. He became a symbol of justice. Joker, however, became an agent of chaos. He snapped when he looked into the abyss (or blinked, as Batman tells Owlman in Crisis on Two Earths). Joker snapped because he believes there is "no sanity clause. There's always madness. You can just step outside and all those dreadful things that happened ... you can lock them away. Madness is the emergency exit".
Joker, in order to prove his point, tries to create "one bad day" for Commissioner James Gordon. I won't discuss how (as the book fans already know, and the newcomers need to see it in the movie), but he wants to practically demonstrate that everyone can snap. If you think deeper, you might even feel slightly sorry for the Joker. He is ashamed of himself. He wants to show that that reaction wasn't unique to him. He wants to prove to others - or maybe more importantly, to himself - that anybody would behave the same under similar conditions. And that is the second act in Joker's saga.
Gordon goes through this experiment, but he doesn't break. He is the bigger man. Gordon still wants to bring in Joker "by the book"... Gordon is incorruptible. He is much better than Joker. He's even better than Batman. Both Joker and Batman were changed when they went to the edge of the abyss. Of course, they are coping with it differently. Joker's coping mechanism made him take the emergency exit of madness, while Batman's coping mechanism killed Bruce and turned him into an obsessive compulsive introvert who has nothing on his mind but vengeance and justice... There is no doubt that Batman's reaction to the situation is a million times more noble than Joker's, but they are similar in the sense that they both have gone to the extreme.
The book also tells us the third act in Joker's life: In the end of the book, Batman offers to help Joker rehabilitate. But Joker is for once wiser than Batman; (i) he knows it's too late, and (ii) Batman needs rehabilitation himself - he is no shape to offer it to someone else. Joker draws an analogy between the two of them and the story of the two guys who want to escape from a lunatic asylum with each other's help. Batman and Joker are like those two guys: there is no escape, and they are not able help each other.
Batman sees the wisdom in Joker's story, and realizes that he has only one course of action left. I confess that I did not realize this when I read the book. The light bulb went off when I listened to the legendary Grant Morrison's interview (on Kevin Smith's Fatman on Batman). To avoid spoilers, I won't discuss it. But if you've seen the movie or read the book, or don't care about spoilers, you could listen Morrison's interpretation at the link below:
The book is simply a masterpiece. It has depth, character, and story, on par with any classic novel (let a lone a comic book). I have read Alan Moore's interviews and how he downplays the importance of the book. But I still think it's a masterpiece. Not because I think I know more than Alan Moore about his work (not even for a single second!), but maybe because I connect with the theme on a personal level.
Now let's get to the movie. As I mentioned, the book is a 48-page comic. The movie also spends around 45 minutes on the material from the book. The producers (Bruce Timm and director Sam Liu) added 28 minutes of new material to the beginning of the movie. The intro focuses on Batgirl and her relationship with Batman. Well, they needed to make the movie longer in order to charge the price that they are charging. But they changed the ending of the movie - the dialogue or action didn't change, but Batman's motivation did. In the book, the decision that Batman makes is a calculated one. He logically chooses that course of action. But the movie, implies that Batman was emotional and impulsive. Speaking of the ending, the director did a very poor job in the last 30 seconds of the movie. The way he framed (or didn't frame) the last sequences, and how he showed a mid-credit scene with Barbara Gordon made the ending anticlimactic.
While we're talking about the direction, I was also disappointed with Joker's singing number. The animation and frame rate were subpar. DC Animation needs fresh blood. We need someone other than Phil Bourassa design the characters, and someone other than Sam Liu to direct. The movies look and feel similar to the previous entries in the DC Universe Animated Original Movies. Not only they are similar, but regretfully the quality is declining with each entry.
There are two things that the movie did impeccably. First, the voice acting was unbelievable. Kevin Conroy, Mark Hamil, and Tara Strong (the same actors from the Batman: The Animated Series) return and they give flawless performances. I mentioned that we need fresh blood in the previous paragraph, but in voice acting, I think nobody is even close to them, and these guys are outdoing themselves every single time. I couldn't imagine it was possible to hear a better performance from the trio after Batman TAS, but they proved me wrong in the first Arkham game (Batman: Arkham Asylum 2009). They proved me wrong again in the subsequent entries. Particularly, Mark Hamil's Joker in Arkham Knight was mesmerizing. Yet here in The Killing Joke, they upped their game again... The second thing that I want to commend the movie on is about the colour pallet that they used: The adopted Brian Bolland's 2008 colour themes (from the 20th anniversary edition of the book), and I enjoyed the colour composition in almost every scene.
The book is a 10/10 (or more accurately, 100/10), but the movie is an 8/10 - mostly because of the last 30 seconds, which ruined the movie for me.