One of the differences between other countries and America is that for America the violence never stopped, and it has been routinely glorified.
And that differentiates us from no other existing nation. See: Falklands War, British Imperial subjugation of India, et cetera.
When Native Americans had something we wanted, we murdered them and took it, over and over again.
That, again, doesn't differentiate America much from other countries in the 1800s; see: Ottoman Empire.
America’s deft use of violence in World War II became our modern claim to moral superiority.
As it did for the U.K. and Russia.
Expansionism via violence is not unique to the United States -- nor is slavery, which is practiced by certain hardcore theocratic governments still today.
And throughout all that time, mass shootings in the U.S. were noteworthy for their rarity, and were almost always carried out by government, not individuals. Instances like the massacre at Wounded Knee, the violence at Kent State.
Another noteworthy thing about those incidents -- they were carried out against disarmed people. Contrast those instances against the Battle of Athens, where former military veterans, armed, forced a corrupt police force to stand down; or against the standoff at the Bundy Ranch more recently, where armed citizens deterred Bureau of Land Management agents from steamrolling over a rancher and his family.
The availability of guns hasn't changed. The greed of our government hasn't changed. What
has changed is the value we place on human life, and of how we regard morality as a whole. I'd bet you just cringed when you merely read the word, 'morality' there, didn't you? Popular culture has been pressing us to regard moral behavior as hokey and embarrassingly old fashioned, and to a lesser degree, that human life is expendable.
Well, this is the result.