Why did Colorado school shooter do it?

... covered in blood, running down the stairs yelling, "Help me, help me, there's a shooter." Leytoldt said she was in yoga class when the incident erupted, and that her teacher told the students to get into a closet. "We were having fun and laughing ...

http://www.wisn.com/news/national/Why-did-Colorado-school-shooter-do-it/-/9373390/23485124/-/120pwo8z/-/index.html?absolute=true