Another post in the thread questioned why this happens in otherwise "civilized and enlightened" societies. My personal response - because we're not - lead me down a train of thought to Adam Lanza, the Newtown shooter. The common thread is a lack of mutual oversight, accountability, and responsibility for others in our communities. Civilization is a wonderful thing that enables any number of deep and extensive value-adds to quality of life for any given individual member of our species, but the specialization inherent in a free society means that people are trusted to attend to their own affairs (and in the case of hierarchy, the affairs of their charges).
As a society, we share responsibility and consequences for the outcomes determined by the actions of individuals. This is the reason that the limits of individual freedom are somewhere in between the capacity and the demonstrated ability to infringe on the rights of others.
What seems to have happened is dereliction of the duty to shape those limits in response to demonstrated infringement in the past. We know - we know - what the status quo wrt, as examples, the penal system and firearms has meant for individual and collective suffering. What you're feeling is the same sense that we're not learning from the past and dooming ourselves to gory, nightmarish repetition.