The NYT has a serious analysis of mass murders among the countries of the world, and the clear conclusion is that the high rate unique to the USA is due to our high rate of gun ownership and relatively lax laws dealing with guns.
The article has multiple graphs and links, but I think it is behind a paywall, so I will provide some key passages:
Why Does the U.S. Have So Many Mass Shootings? Research Is Clear: Guns. (Published 2017)
Americans advance a lot of theories for why they have so many more gun deaths than other countries do. The answer is lying in plain sight.
www.nytimes.com
The article has multiple graphs and links, but I think it is behind a paywall, so I will provide some key passages:
Americans make up about 4.4 percent of the global population but own 42 percent of the world’s guns. From 1966 to 2012, 31 percent of the gunmen in mass shootings worldwide were American...
a country’s rate of gun ownership correlated with the odds it would experience a mass shooting. This relationship held even when he excluded the United States, indicating that it could not be explained by some other factor particular to his home country. And it held when he controlled for homicide rates, suggesting that mass shootings were better explained by a society’s access to guns than by its baseline level of violence.
If mental health made the difference, then data would show that Americans have more mental health problems than do people in other countries with fewer mass shootings. But the mental health care spending rate in the United States, the number of mental health professionals per capita and the rate of severe mental disorders are all in line with those of other wealthy countries...
countries with high suicide rates tended to have low rates of mass shootings — the opposite of what you would expect if mental health problems correlated with mass shootings.
Whether a population plays more or fewer video games also appears to have no impact. Americans are no more likely to play video games than people in any other developed country.
Racial diversity or other factors associated with social cohesion also show little correlation with gun deaths.
United States is not actually more prone to crime than other developed countries, according to a landmark 1999 study by Franklin E. Zimring and Gordon Hawkins of the University of California, Berkeley. Rather, they found, in data that has since been repeatedly confirmed, that American crime is simply more lethal. A New Yorker is just as likely to be robbed as a Londoner, for instance, but the New Yorker is 54 times more likely to be killed in the process.