I've always liked movies with evil children. So now I'm looking for mangas with evil kids. Boy or girl does not matter. Any genre/demographic is fine but please no parody characters or kids who are evil for the sake of comedy.
The character needs to be a child (max 14-15) and be evil/sadistic/twisted/psychotic. I don't want children who are simply ruthless, that's not enough. The character needs to be an actual villain or antagonist and get enjoyment out of killing and hurting people, though they can still be treated somewhat sympathetically.
Kami no Kodomo - It really is genuinely disturbing, about a young sociopath serial killer. It's rather graphic and the character is truly loathsome. He is 17 at the end, but most of it takes place when he is younger.