My recollection is that the very first incidence of mass murder was the sniper from the University of Texas tower in 1966. Illegal drugs, immoral sex, and hardcore violence began in that decade and our society has been swirling down the toilet ever since.
http://en.wikipedia.org/wiki/Howard_Unruh
Killed 13 in September 1949. Somewhat similar in psychological state as some of the more recent shooters.