What leads to School Shootings?

I read an article called "Boy,14, Seized in Plot to Open Fire at School" in the New York Times by Jon Hurdle. It was about a fourteen year old boy plotting to start a "Columbine style" shooting at Plymouth Whitemarsh High School. He was going to start it supposedly because he was bullied around at the school. In his room the police found a rifle, imitation guns, homemade grenades that were operational, and a DVD on the Columbine Shooting. The boy was arrested by a tip from a boy who the other was trying to make join him in the shooting.

What leads to a school shooting? I think it is partially because of the way the person is treated during their time in school. Also I believe that a lot of it has to do with the stress of the average day on top of pier pressure. Why do these people kill the innocent bystanders at the schools? I do not have any clue to why they would take out their anger on them. I can't understand why this happens but these shootings usually lead to the shooters suicide. This makes me wonder do these people kill others to get back at the world before they kill themselves and try to ease their pain.