These might be handy in the future.
Database about historical violence: cjrc.osu.edu/researchprojects/hvd/europe.html.
On long term trends in violent crimes: Eisner, 2003, Long-Term Historical Trends in Violent Crime. Crime & Just, 30, 83-142. [PDF].

Steady flow of mass shootings between 1982 and 2012: http://www.motherjones.com/politics/2012/07/mass-shootings-map