Question about the shooter(Michael Patrick)

Does anyone know if theres a background like why he did what he did....maybe someone who read the book or was it never an issue(dont get the feeling that i think the movie or book would be about his motivation in any way ,I'm just curious about some possible extra background story ,because it is never shown that he was bullied....
anyone?????

The only thing worse than a film critic.....is the average loser that tries to act like one!!!