no one likes to admit that paediatric abnormal psychology is a thing.

Reading up on child sociopathy/psychopathy is sobering. They start exhibiting problems almost as soon as they're able to interact with other children. By the time they're 6 they're torturing animals/insects. By age 10 they've attacked or threatened a child or adult with a knife. By 13 they're in a detention facility of some kind. There's virtually no hope for them other than a lifetime of imprisonment.

And they can come from perfectly normal families, with perfectly normal siblings.