The Columbine Massacre: In the Killer's Mind

It happened on April 20th, 1999, at the Columbine high school in Colorado. Two students arrived armed to the teeth. They proceeded to execute 13 people and wound dozens of others before committing suicide. The people of the state of Colorado were stunned, and so was the rest of the world. What strange motivations led them to commit this senseless act of violence? Could anything be done to stop similar incidents?