I think you are underestimating the lethality of firearms available from the turn of the 20th forward, as well as explosives and other tools for killing en masse. The worst school killing was with explosives, not a gun. The Camden walk of death in 1949 involved only a handgun. The mobsters of the 20s and 30s had lots of automatic weaponry (suggesting that such could have been found. Yet the rash of mass shootings we now experience isn't simply a question of accessibility of semi-automatic weaponry. Such weapons have been available for the last 50-60 years. But only in the last 30 or so do we see these kinds of events becoming comparatively frequent.
Why now and not in the turbulent 60s? Why is 2001-2018 so much more rife with such events than was 1972 - 1990? Even if I grant that semi-automatic weapons are more accessible now than a century ago, it still does not address why it is more common now than it was in the 1980s? Your position on gun control cannot be the sole answer (setting aside the Constitutional issues currently under debate). There is more to it than that.
To be fair, the only colleague I have known personally to have been murdered was murdered by his ex wife. I was using colleague in the larger sense with my earlier comment. I did learn of three different graduate faculty (all STEM types) who were murdered by students after awarding them failing grades in graduate school. Two shootings and one death by ballpeen hammer.
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