Just the whole concept of it. It's like Oz meets Elfen Lied meets more insanity.
Episode 1: within five minutes of the first episode you get to see a classroom of middle school kids mutilated into shredded corpses covered in glass, concrete, and their own dismembered bones and blood. And then you see Mimi's bloadsoaked head clutched in the Red Man's hand like a trophy... with the most horrified look on her face. What does that imply? She saw the whole thing and begged not to be killed. But the Red Man found it more pleasing to his sick gullet to just put her out of her misery.
And then comes the trial. Not only is Ganta a helpless little kid under the mercy of an iron-fisted court, he has nobody with any moral fiber to defend him. On top of that, he's seamlessly impersonated and incriminated with evidence he's an insane killer. After panicking at the sight of everyone ready to convict him, he looks like he's lost his mind... which of course, causes the grief-stricken parents of the victims to think he has no remorse (for a crime that wasn't even his doing), and beat the hell out of him. Next, his corrupt bastard lawyer deserts him and he gets a life sentence. Then, of all things, he gets egged. Finally, he's placed in a jail that can only be described as Satan's playhouse. Think you're having a bad day?? Like hell you are compared to this shit.
The dog race from episode 2. The fact that the audience were watching real people die.