Most mass murderer's world wide are male -
"The collection of general statistics and characteristics about mass murderers is informative. We find that the mass murderer is almost always male"
Profiling Mass Murderers
The killers: "Half of the cases involved school or workplace shootings (12 and 19, respectively); the other 31 cases took place in locations including shopping malls, restaurants, and religious and government buildings.
Forty four of the killers were white males. Only one of them was a woman. (See Goleta, Calif., in 2006.) The average age of the killers was 35, though the youngest among them was a mere 11 years old. (See Jonesboro, Ark., in 1998.) A majority were mentally ill—and many displayed signs of it before setting out to kill. Explore the map for further details—we do not consider it to be all-inclusive, but based on the criteria we used we believe that we've produced the most comprehensive rundown available on this particular type of violence. (Mass shootings represent only a sliver of America's overall gun violence.) For a timeline listing all the cases on the map, including photos of the killers, jump to page 2. For the stories of the 151 shooting rampage victims of 2012, click here, and for all of MoJo's year-long investigation into gun laws and mass shootings, click here."
A Guide to Mass Shootings in America | Mother Jones
But for some reason, there are way more mass murders in the US than other developed nation:
"As a 2011 United Nations report notes, America has a “relatively high homicide rate compared to other countries with a similar socio-economic level,”
"For 15-year olds to 24-year olds, firearm homicide rates in the United States were 42.7 times higher than in the other countries.”
Mass murder, shooting sprees and rampage violence: Research roundup – Journalist's Resource: Research for Reporting, from Harvard Shorenstein Center
Now, the question is why?