I specified non-political mass shootings by individuals, not mass killings in general. Obviously mass killings in general existed prior to that, and there were a few mass shootings prior to them becoming extremely common. The event that caused them to become common was Columbine, not the Charles Whitman mass shooting or any other historical mass shooting which had occurred rarely and sporadically until that point.
I specified non-political mass shootings by individuals, not mass killings in general. Obviously mass killings in general existed prior to that, and there were a few mass shootings prior to them becoming extremely common. The event that caused them to become common was Columbine, not the Charles Whitman mass shooting or any other historical mass shooting which had occurred rarely and sporadically until that point.