Not only did he choose the most defenseless targets he could find, Adam Lanza shot himself as soon as he knew the cops were coming. See Gov.: Gunman shot self as 1st responders closed in.
The guy was deeply disturbed, but a proper diagnosis is something for the shrinks to excavate from the rubble of his pathetic life. However based on what the reporters are saying, he was not a brave person. When he heard the police, his fear kicked ink, and he checked out.
But it took the arrival of the police. And as the saying goes, when seconds count, police are just minutes away. So with that in mind, and while lawmakers get ready to slap some new emotion driven laws on us, let's do some speculating. If anyone at the school -- a teacher, a janitor, a visitor -- had advanced toward him pointing a gun the carnage might have ended earlier.