Mass killings, and serial killings, are in decline. Murder overall, in fact, is in decline except for in some (Blue) cities like Chicago and Detroit which nobody seems to get excited about very much.
Everybody is seeking scapegoats for mass killing, especially when it's white suburban kids instead of the slow mass murder of inner city black kids, and everybody hops onto the hobby horse they want to ride. Movies and video games, firearms, bomb ingredients, the mentally ill, etc.
The problem is that these mass events are so rare as to be utterly upredictable. We cannot put an armed guard at the door of every classroom. And trust me, you cannot lock up, indefinitely, every paranoid or angry person who has thoughts of killing people. You'd have to lock up half the commenters at Daily Kos. Thoughts about killing, like thoughts of suicide, are very common.
Furthermore, mass killers like Timothy McVeigh and serial killers like Ted Bundy were not even mentally ill in any usual sense. Evil, not ill. Not to mention Nidal Hasan, the Fort Hood killer. A military Psychiatrist, no less (and a presumed Jihadist). Or the 9-11 killers.
Terrible events are black swans and probably not preventable. Dangerous people rarely seek help with their problems anyway, and criminals ignore laws. Nobody who is hell-bent on mass killing is likely to tell anyone.
From the NYT yesterday, Focus on Mental Health Laws to Curb Violence Is Unfair, Some Say.
Here is Top 10 Myths About Mass Shootings
Tracked: Feb 02, 13:00