After the Charleston massacre, President Obama made this comment: “the unique mayhem that gun violence afflicts upon this nation.” The three worst public mass shootings and nine of the worst 13 public mass shootings occurred outside the US. Of these cases, nine occurred in developed or “advanced” countries. The worst public mass shooting are as follows:
1) Utoya, Norway, July 22, 2011: Anders Behring Breivik used a gun to kill 67 people and wound 110 others. Still others were killed by bombs that Breivik detonated.
2) Sousse, Tunisia, June 26, 2015: Seifeddine Rezgui, a college student, used a gun to kill 39 people at a crowded tourist beach resort at the Imperial Marhaba Hotel.
3) Port Arthur, Tasmania, Australia, April 28 and 29, 1996: Martin Bryant killed 35 people and wounded another 23 at a popular tourist site, the historic Port Arthur former prison colony.
4) Blacksburg, Virginia, USA, April 16, 2007: Seung-Hui Cho killed 32 people and wounded 17 by gun fire at Virginia Tech University.
5) Hebron, Israel, February 25, 1994: Baruch Goldstein killed 29 and wounded 125 Muslim worshippers in an attack on the Cave of the Patriarchs.
6) Newtown, Connecticut, USA, December 14, 2012: 20-year-old Adam Lanza killed 26 people at the Sandy Hook Elementary School.
7) Killeen, Texas, USA, October 16, 1991: George Hennard crashed his pickup truck into the plate glass windows of the Luby Cafeteria and then shot 23 people to death.
8) San Ysidro, California, USA July 18, 1984: James Huberty shot and killed 21 people and wounded another 19 at a McDonald’s restaurant.
9) Erfurt, Germany, April 26, 2002: Robert Steinhäuser, a recent graduate of the Gutenberg-Gymnasium in Erfurt, Germany, killed 18 people.
10) Near Stirling, Scotland, UK, March 13, 1996: Thomas Hamilton killed 17 people at the Dunblane Primary School.
11) Hungerford, UK, August 19, 1987: Michael Robert Ryan killed 16 people, but they occurred at several locations.
12) São Gonçalo do Amarante, Natal, Brazil, May 21, 1997: Genildo Ferreira Do Francais killed 15 people in a small northern Brazilian town.
13) Winnenden, Germany, March 11, 2009: 17-year-old Tim Kretschmer 15 killed at the Winnenden School.
Other cases: The Zug Switzerland shooting left 14 people killed in September 27, 2001. The 1966 University of Texas shooting left 14 people dead (wife and mother were killed through other means). Edmond, Oklahoma shooting at a post office left 14 people killed on August 20, 1986.
UPDATE: Woo Bum-kon killed 56 people a over two days in South Korea (April 26-27, 1982). It is hard to include him in the above list because there was no large number of people killed in any one single public place, but instead he would kill one or two people in a lot of different public places. He also used grenades to kill people.