In ritually determined cultures, traditional cultures, in almost all cases you have some version of a Bacchanal. Christmas (which by the way derives from Christ plus Mass; holiday, in turn, derives from Holy Day) itself was layered on to the Saturnalia, which was a multiday party where everybody let it all hang out.
All people–which by definition includes people organized into a social order–need to yell shout and scream sometimes. You need to “lose it”. You need color, wildness, chaos. Our own Halloween is a pale reflection of this. Mardi Gras in New Orleans, and the Caribbean, and Carnival in Brazil and elsewhere are examples. In India they have Divoli. Even the Muslims have a party at the end of Ramadan. I suspect Japan and China have their historical examples.
I was thinking in particular of the Tarahumara Indians (Raramuri, approximately, in their own tongue, meaning the “running people”) are very calm and reserved normally, but when it’s time for a run, they get high as kites, drink heavily, fight, screw each others wives, pass out, then get up and run 50 or 100 miles, which of course is also an outlet for energy.
These mass shooters, it seems to me, are exercising and exorcising energies that have been pent up for long periods of time and which they had no other way to get out, no other socially condoned pathway, no route to letting their hair hang out in public, and not feeling the fear of rejection.
If Adam Lanza has expressed himself in small ways repeatedly, and not been rejected, I think the shooting would not have happened.
I will add, though, too: in past ages, it simply was not possible to interact with artificial human beings, which is what we are doing when we watch movies and TV and play video games. You were either fully alone, or with actual humans. I think this emphasized tribalism–the past was not less violent–but also a sense of continuity with at least your own group.