The guy who scratches his fingernails off trying to get out of the room... and fails.
There's that scene, and then there's the sight of the Joker and his henchman handing out cotton candy to Cub Scouts at a fair. Later, as Batman and Robin are Bat-glidering into the fair, we see from their perspective the same Scouts, all dead from the poison in the candy and sprawled around like an image out of Jonestown. The Joker's lines under Ax-Crazy on the main page are taken from this scene, for added horror.
The fight between Abner and Robin ends with Abner choking Robin from a roller coaster car in motion. However, he isn't paying attention to his surroundings and is decapitated by a low overhang. The book makes it clear Robin is clearly traumatized by the experience. The movie is even worse in which Robin is more directly responsible, knocking Abner into the coaster's gears where his shirt gets caught and his head is crushed. The Gory Discretion Shot is nullified by the horrific sounds of grinding and splatter as he dies.
Joker's face as he dies.
Joker KEEPS this face even as a smouldering skeleton.
Batman (inner monologue):Stop laughing.
The flying robot kid things. "Eeugh yourself, bitch."
Gordon invokes this in his internal commentary about seeing the Mutant leader kill the mayor.
I hear a nervous giggle and an animal growl. I hear handcuff links snap. I see something I'll take to my grave.
The film goes more into depth around the Joker's rampage in the Tunnel of Love, following him as he shoots everyone he comes across, playfully exclaiming "Excuse me! Pardon me! Sorry about that! My bad" and the like. The casual glee he gets in just shooting as many people as possible is very, very disturbing.
Joker's television massacre was nasty enough in the comic but the movie goes great detail of the audience panicking and nastier joker-smiles. Plus Joker kills his doctor not with one of the dolls snapping his neck but by slitting his throat with a broken mug.
Let's just say pretty much every scene with The Joker. Michael Emerson makes him utterly terrifying and he does it without ever raising his voice or even showing much emotion beyond amusement. It just shows how completely removed from any humanity The Joker truly is.
In the orginal script, Batman was to fake his death by using a hospital tube needle to gouge out a pill hidden in the flesh of his forearm. The scene was described as bloody, as painful and as having Bruce enjoying it.