What is the theme in Columbine by Dave Cullen?

The need to understand Dylan's and Eric's actions is the major theme of the story. The author pointed out repeatedly that people wanted to know why Dylan and Eric walked into the Columbine High School and killed thirteen of their classmates. The short answer is that they were psychopaths filled with rage, wanting to do a great deal of damage to the world but settling on the attack on the school. The boys had set out with a plan to bomb the school and to shoot those who were fleeing the scene. The bombs didn't go off so they went inside with the intention of simply killing whoever came into contact with them.