Yesterday, I looked up a bunch of mass murder incidents and made a spreadsheet of the data.
(Just out of curiosity and because I'm a total nerd.)
The data below is some of what I discovered --
Age of Suspect (average) - 31
Age of Suspect (mode) - 28
Mental Illness / Brain Disease - 8
1949 "Walk of Death" Shootings - Camden, NJ
1966 University of Texas Bell Tower Shooting (brain tumor) - Austin, TX
2005 Red Lake Shooting - Red Lake, MN
2008 Northern Illinois University Shooting - DeKalb, IL
2012 Sandy Hook Shooting, Newtown, CT
2012 Aurora Theater Shooting - Aurora, CO
2017 Sutherland Springs Church Shooting - Sutherland Springs, TX
2018 Thousand Oaks Bar Shooting - Thousand Oaks, CA
Unknown / Inconclusive - 7
1984 San Ysidro McDonald's Massacre - San Diego, CA
1986 Edmond Post Office Shooting - Edmond, OK
1999 Columbine School Massacre - Columbine, CO
2006 West Nickel Mines School Shooting - Bart Township, PA
2007 Virginia Tech Shooting - Blacksburg, VA
2017 Vegas Concert Shooting - Paradise, NV
2018 Stoneman Douglas School Shooting - Parkland, FL
Injustice Collectors - 6
1927 Bath School Bombing - Bath, MI
2009 Binghamton Shootings - Binghamton, NY
2009 Geneva County Shootings - Geneva County, AL
2013 Washington Navy Yard Shooting - Washington, D.C.
2014 Marysville School Shooting - Marysville, WA
2018 Santa Fe High School Shooting - Santa Fe, TX
Terrorism - 5
1995 Oklahoma City Bombings (domestic) - Oklahoma City, OK
2016 Pulse Nightclub Shooting - Orlando, FL
2009 Fort Hood Shooting - Killeen, TX
2013 Boston Marathon Bombings - Boston, MA
2015 San Bernardino Shooting - San Bernardino, CA
Hate Crimes - 5
1991 Luby's Shooting (women) - Killeen, TX
2015 Umpqua Community College Shooting (women) - Roseburg, OR
2015 Charleston Church Shooting (racism/white supremacy) - Charleston, SC
2018 Pittsburgh Synagogue Shooting (racism against Jewish community) - Pittsburgh, PA
2019 El Paso Walmart Shooting (racism/white supremacy) - El Paso, TX
Anger/Revenge/Retribution - 4
1990 Happy Land Fire (argument with former girlfriend and jealousy) - New York, NY
2006 Capitol Hill Massacre (resentment toward "the raver" lifestyle and sexual freedoms) - Seattle, WA
2012 Oikos University Shooting (revenge for school expulsion) - Oakland, CA
2019 Virginia Beach Shooting (disgruntled worker) - Virginia Beach, VA
Financial Loss - 1
1999 Day-Trading Firm Shootings - Atlanta, GA