What causes school shootings? I'm not sure, but I think it's a switch in the perps noggin that just "flips" on or off by some stray neuron firing. This guy was obviously mentally damaged.
In some countries and cultures, they would "eliminate" all the perp's children if he had any. Not to exact an "eye for an eye" mind you, but because they figured out that they could eliminate the "sicko defect" which he obviously carried. They don't want it passed on. That's a bad policy. However, I do give them credit for trying.
Just my opinion...
One thing is for certain, and is readily observable if one studies the last decade's worth of these incidents -- when one incident happens, others quickly follow.
The reason for this is plain: The media plasters stories of the initial event all over the front page and TV screen for days on end, thereby pushing any borderline "fence sitters" over the edge. Whether intentional or not, this effect is seen in almost every instance.
So in a sense, the media itself, in making such a spectacle of tragedy, sets the stage for the next bloodbath.
As my brother (A sociology major) said years ago, "the media keeps the crazies thinking about it".