Sarah, to my knowledge you were never a teenage boy, so you'll probably just never understand that physics class was all a means to work with ballistics and such. I myself made a pneumatic cannon for a project and went launching golf balls all over the soccer fields. It was AWESOME.
Face it: the kid is a narcissist and sociopath. "(H)e felt his family had betrayed him..." He sees the world as revolving around him with his parents as properly conceived as satellites whose role was to orbit his center of gravity. Remember, he was accused of date rape. While that was not provable, his apparent desire to knock off hundreds of people in an explosion makes rape of a single person seem more than just slightly plausible. A sociopath can't empathize with the feelings of others. I'd say that planning to kill noncombatant people, including children, en masse probably allows us to apply the definition to him.
http://www.youtube.com/watch?v=FwNf-3VHRhE