Guys, I know I'm not a detective, I know I don't know much about the story, but I do believe I might have figured out, MAYBE, why the man shot up the elementary school.
I read somewhere earlier today, that a parent with a teaching job who has a child of her own, the child will begin to feel less and less important in the parents life, and begin to feel like the students are more important. The shooters mom was a teacher. He never felt important enough from the get go. His mom has been "pushing him away" because of the students, and he's had enough of it. So, he decided to stop feeling that way through the death of his mother, for making him feel that way, and through the death of the students, for making his mother ignore him.