I posted this earlier:
"Mass murder is defined as “the sudden, intentional
killing of more than one person in the same location and at the
same time, usually by a single person” (Palermo & Ross, 1999,
p.8). Turvey (2008) notes that the problem of mass murders is
not unique to the United States. However, Hamamoto (2002)
argues that the United States produce most of the world’s mass
murderers because of a “blow back” by civilians scripting
violence
in a hyper-militarised America which started with the
increasing military adventures after World War II. Research on
mass murder is relatively limited when compared to other
forms of multiple homicide (Bowers, et al, 2010), yet several
authors have identified descriptive characteristics, patterns, and
typologies that differentiate mass murder from other forms of
multiple murder and from murder in general (Fox & Levin,
2012, Bowers, et al, 2010, Bartol & Bartol, 2011).
The rich countries that have less mass murders than the US is from other studies that show that:
"The U.S. has far more gun-related killings than any other developed country"
Chart: The U.S. has far more gun-related killings than any other developed country