Again, people have been walking into schools and shooting people since the 40's (Before then, but lets just say the 40's.) The difference is that these people tended to kill 1 or 2 people before killing themselves. The difference in the power of the weapons doesn't explain the lower body count, because the weapons used were certainly powerful enough to cause far, far more damage. The fact was that in most of these cases, the men didn't even try to kill anyone besides their target and then themselves.
The amount of deaths from these events has climbed as media saturation has become more intense. I'm completely on board with the forensic psychologist in one of the previous posts pointing toward the media's fetishistic coverage of the killer creating an appeal to do more damage before the crazy person kills themselves. Anyone who has read some Durkheim will understand that the media's coverage is ritualistic (Even how they present the story is almost like ritual now) and such large scale, unifying rituals are extremely inviting for certain elements in society (Everyone wants to be a big sports player? It's kind of like that, but for crazy people.) Combine that with lower access to quality mental (And general) healthcare, along with a less prosperous/hopeful view of life (Which almost always leads to higher crime rates) and you have a perfect brew for crazies.
I think, on the whole, we can prevent far more violent deaths (Maybe not mass shootings, but violence in general) by improving the quality of life for normal people--If we're not having that debate, about providing for basic human needs,beforegun control, then we're obviously only interested in how things look, rather than what will really help.