Complete Monster: The Joker once again defines himself as a sadistic lunatic. Joker opens up the film by brutally beating the young Jason Todd, the second Robin, to a bloody pulp with a crowbar, before leaving him to die in an explosion he sets off. After Joker is released by a reluctant Black Mask from Arkham Asylum to kill Red Hood, Joker demonstrates his random violence by slitting the throat of one of his guards and shooting the other four dead, and soon after backstabs Black Mask by gagging him and the members of both his and Red Hood's gang, intending to burn them all alive for the purpose of getting Red Hood's attention. Once it's revealed Red Hood is in fact a resurrected Jason Todd and Batman turns the tides against Red Hood in the final confrontation, Joker tries to pin Batman down for the purpose of dying with both his arch-rival and Red Hood in an exploding building, absolutely ecstatic that he gets to die getting everything he wants. Even the hardened kingpin Black Mask is wary of the Joker, and Batman himself reveals he contemplates killing the Joker every day of his life for the clown's atrocities, only relenting to prevent himself from becoming just like those he opposes. As true to his darker interpretations, the Joker was a psychopath motivated by random whims at best, and a desire to hurt Batman at worst.
Counterpart Comparison: A lot of people noted Red Hood's resemblance to Deadpool when the first trailers went online. The similarities are even more marked in the actual film, where the character behaves near-identically to Deadpool in the latter's earlier appearances.
Foe Yay: The Joker interrupting Batman's Tear Jerker speech by saying "aw, so you do think about me!" In the Joker's typically disturbing fashion, he says this while Batman is saying that not a day goes by where he doesn't think about killing the Joker.
Ra's al Ghul. Despite his being a ruthless supervillain who, as Batman says, has never shied away from shedding blood, his deep remorse and sorrow over Jason's fate, resurrection, and subsequent rampage are clearly heartfelt and genuine.
Memetic Mutation: Variations of this piece of dialogue, swapping out Batman, Jason and Joker for other characters/parties:
Jason Todd: Bruce, I forgive you for not saving me. But why, why on God's Earth...is he [Joker] still alive?!
Moral Event Horizon: The Joker crosses his in the film's opening scene, as he savagely beats Jason, then leaves him to die.
Jason killing the Joker would not be this. Him handing Batman the pistol and telling him he has to do it qualifies to some.
Strawman Has a Point: Like in the comic, Red Hood raises a good point on the naivety behind the idea that someone like the Joker can continue to walk the earth even though he'll continue to kill thousands. And the fact that it's a logical fallacy to say killing the Joker would make Batman accept killing any criminal, when Joker has been the one villain who has never shown any capability of possible redemption in any form, unlike the rest of the Rogues Gallery. He's also right about Batman not being able to stop crime — how long has Batman been trying without any success? And sometimes he even manages to make it worse.
Reactions to John DiMaggio's casting as Joker were... mixed, to say the least. The release of the character models featuring a somewhat burlier Joker eased concerns somewhat. And then the film was released and a lot of those who doubted he could pull off the role did a complete 180.
Audiences were also skeptical of Jensen Ackles playing a lead character, as he had never done voice acting work prior to this film. Again, he proved them wrong.
And anyone other than Kevin Conroy as Batman raises suspicions. But Bruce Greenwood did a good job, especially as a Batman who's supposed to be a little older than we normally see him. Greenwood would later reprise the role for Young Justice where he recieved even more praise as a Batman who's decidedly a very good and encouraging mentor.