What are the motifs in Columbine by Dave Cullen?

The need to blame someone is a motif in the story. Patrick and Dylan killed themselves after killing twelve other students and a teacher. They'd intended for the death count to be much higher and had actually expected to be killed by police but committed suicide when that didn't work out. An attorney for the parents of one of the boys' parents warned them immediately after the shooting that the public would need someone to blame. That proved to be true as many people believed the parents of Patrick and Dylan were responsible and should be held accountable.