Laws are basically just threats. Some threats are "officially" sanctioned. Others are unofficial. You could run a society just fine with unofficial threats. Personally I think it's possible to run a society without threats at all... kind of like it being possible to raise kids without threats - maybe not easy, but possible.
Come to think of it, mass shootings, like domestic terrorism, and being forced to deploy riot police, is just another leading (or maybe lagging) indicator of a failed state - just different forms of social unrest.