Well, I work in the justice system, and I've heard a lot of anecdotal stories about this person and that person who "just snapped," and you often hear the neighbors of the mass murderer on TV saying, "I can't understand it, he was such a nice boy." But when you actually get the parties into court, you find that that is almost never the case.
Put people under oath, and subpoena medical records, and you usually find that somebody like this has a long history of mental problems, run-ins with the law, or both. I would not be at all surprised to hear that this boy has had numerous previous encounters with juvy. Those records are often sealed or confidential, but in a case like this they will come out. In fact the defense will probably use them to try to mount an insanity/diminished capacity defense.
I often hear the "just snapped" argument in support of gun control or some other repressive plan. Think about it -- if people can "just snap", would you trust ANYbody with a firearm . . . makes a good argument for gun control, I suppose, but if it were true there would be daily murders in the streets as people "snapped" over this and that.
What's going on is usually frantic denial on the part of everybody who knew the kid and didn't connect the dots and is now feeling guilty that they didn't speak up.
He knew how to pull a trigger, he knew how to hunt. Are we going to make something out of that?
I am sure we will hear someone say he lost his temper at such and such a time or place, but I don't think THIS could have been predicted, but someone will dispute that.
PS..I am NOT defending his actions in anyway..