Diamond says kids need boundaries and limit-setting to develop a sense of morality and to learn appropriate behavior. Without that attention or discipline, kids may get the message that they aren't worth the effort in the first place. And that can play a role in developing what Diamond refers to as "psychopathic narcissism," or narcissism gone to an extreme. He cites O.J. Simpson as an example.
It's what happens when the child doesn't get certain needs met when they're growing up or is overindulged in certain ways. It's feeling like they have the right to intrude on other people's lives, to manipulate other people - it's all about them. And they tend to not have much of a sense of conscience or guilt about what they do because they're so self-serving.
In other words, the ability to commit acts of evil doesn't just come out of nowhere ...
Behavior doesn't just happen out of the clear blue. There's always some prelude to it.
To get Diamond's views on the Juran van der Sloot specifically, check out his post on Evil Deeds. And if you want even more info on the genesis of evil, he's also written a book on it.