The motif of selfish and abusive parents
Cameron, Len and Carla are united by a single feature: All their parents are nightmarish. Cameron's story is the most representative of the three, because his story indicates the basic dysfunction that causes children to develop pathological relationships with others: A parent who has plenty to offer, but keeps it all for themselves, leaving the child in a position where his/her psyche is being shaped by young, traumatic experiences of mistrust, mistreatment, and even hatred. The novel's core idea seems to be that it was the parents of the traumatized, disenfranchised children who planted the seeds which grew into terrifying acts of violence against innocent people.
The motif of rejection
The children are commonly rejected. Carla is a goth, which makes her different from "normal." All three are generally rejected by their own parents, and ultimately Len's act of violence is his rejection of life, love, and existence.
The symbolic handwriting
The diary of the killer is filled with seriously bizarre content. For one, his handwriting is barbaric. Also, his sentence structures and word choices indicate that he might be bad at reading and writing, meaning that he is also suffering from troubles in school, which might explain why is rage becomes localized to the context of his academic failure and his social failure.
Cameron as a foil
A foil is a character whose qualities contrast the main character. Cameron's value as a foil is that Cameron too is rejected for unfair reasons (racism even), and his parents mistreat him, but the effect is not that he wants to kill anyone.
The diary as a symbol
The diary itself seems to represent the real life blog of the Columbine killer, because the tone and the iconoclastic use of language is shared between them, and the flavor of their hatred is undeniably similar. In other words, the story is fictional only by costume. It's intent and psychology are horrifying and unfortunately factual.
