What they are not...
They are not new.
They are not uniquely American
They are not as frequent as we are led to believe (and yes, even one it too many)
They are not preventable by any law.
The absolute most any law can do is make it slightly more difficult for a given individual to get certain tools, so a law banning X could well result in the killer using Y, which might be even more deadly in effect. (can't easily -or cheaply- get an "assault weapon, so use a bomb, or gasoline and a match....etc..)
They do not happen where people are both armed, and willing to fight back.
(even on an Army base, there are only a handful of armed police. Regular troops do not have their arms (or if they do, they do not have ammunition for them). They are all locked up in storage until issued for use (including training).
Physically, we are animals, specifically mammals. And it is an observed fact that among mammals, certain species have individuals that, for no obvious reason run "amok". We can't talk to the animals to learn why, and when they kill themselves (or are killed) we can't talk to the killers to learn why. But it is clear that it does happen, everywhere there are humans, with no clearly discernable pattern.
We cannot read minds (and if we could, would it be ethical?). There is no way to determine who is going to snap, or when. Some give hints, or clues, but by no means all do. And recognizing those clues for what they are, BEFORE a violent act is committed is seldom possible.
Judging what is actuall a creditable threat to society and what is just a harmless raving is a really tough call. Particularly with our right to free speech, and the sloppiness of our language. Do we throw sports fans in jail for years/decades because they shout "Kill 'em!!!" at the game? No, we don't.
But we do investigate someone who says "I would like to shoot the President!" And if the threat is deemed creditable, you bet we take action.
But what about the people who say nothing? Or even LIE?...at least one of the Columbine killers had "peace, and love, diversity, and lets all get along" type messages all over their webpage. Not very reflective of what they actually felt, or did, though.
There is no easy fix, and world history seems to show that there is really no "fix" at all, at least not so long as mankind remains what we are. The only thing I am certain about, is that no gun control law (or any kind of law) will prevent it happening again, somewhere, sometime.
And the people telling us that it will, or that "if it saves just one..." are either woefully ignorant, or deliberately lying to us, and themselves.