Half the firearms in the entire world are in the United States. The rate of murders by gunfire in the United States is almost 20 times higher than the average rate in 22 other populous, high-income countries where the frequency of other crimes is about the same. There is clearly a connection between these two facts, but it is not necessarily simple cause-and-effect.
Here's one reason to suspect that it's not that simple: The American rate for murders of all kinds shooting, strangling, stabbing, poisoning, pushing people under buses, etc. is seven times higher than it is in those other 22 rich countries. It can't just be guns.
Steven Pinker, whose book The Better Angels of Our Nature is about the long-term decline in violence of every kind in the world, is well aware that murder rates have not fallen in the United States in the past century. (Most people don't believe that violence is in decline anywhere, let alone almost everywhere. That's why he wrote the book.) And Pinker suggests an explanation for the American exception.
In medieval Europe, where everybody from warlords to peasants was on his own when it came to defending his property, his rights and his "honor," the murder rates were astronomically high: 110 people per 100,000 in 14th-century Oxford, for example. It was at least as high in colonial New England in the early 17th century.
By the mid-20th century, the murder rate in England had fallen more than a hundredfold: in London, it was less than one person per 100,000 per year. In most Western European countries it was about the same. Whereas the U.S. murder rate is still around seven people per 100,000 per year. Why?
Pinker quotes historian Pieter Spierenburg's provocative suggestion that "democracy came too early" to America. In European countries, the population was gradually disarmed by the centralized state as it put an end to feudal anarchy. Only much later, after people had already learned to trust the law to defend their property and protect them from violence, did democracy come to these countries.
This is also what has happened in most other parts of the world, although in many cases it was the colonial power that disarmed the people and instituted the rule of law. But in the United States, where the democratic revolution came over two centuries ago, the people took over the state before they had been disarmed and kept their weapons. They also kept their old attitudes.
None of this explains the specific phenomenon of gun massacres by deranged individuals, who are presumably present at the same rate in every country. It's just that in the United States, it's easier for individuals like that to get access to rapid-fire weapons. And, of course, the intense media coverage of every massacre gives many other crazies an incentive to do the same, only more of it.
But only one in three hundred murders in the United States happens in that kind of massacre. Most are simply due to quarrels between individuals, often members of the same family. Private acts of violence to obtain "justice", with or without guns, are deeply entrenched in American culture, and the murder rate would stay extraordinarily high even if there were no guns.
Since there are guns everywhere, of course, the murder rate is even higher. But since the popular attitudes to violence have not changed, that is not going to change either.