The question is, how do we know when that moment will be?   

As cultural systems begins to dissolve due to political clashes and economic decline the real evil tends to slither out of the woodwork.  It happens slowly at first, then all at once.  A sure sign of accelerating collapse is the growing prevalence of psychopaths and psychopathic behavior in the open.

The US appears to have entered the middle stages of such a collapse with many sociopaths and psychopaths beginning to feel that they might be able to act out their worst impulses without consequences.  They are beginning to test the waters to see what they can get away with.

In the past ten years there has been a dramatic uptick in mass violence and theft.  With the advent of social media it is now easier than ever for spontaneously planned riots to form with little warning, and in most cases these mobs are random in who and what they attack.  They might organize in the name of politics or activism, but they tend to lash out at whatever targets are closest or easiest rather than the people they blame for their travails.

In most cases these events result in simple property destruction in urban areas, but more and more there has been an underlying and aggressive impulse to hurt people.  There will come a time very soon when the the goal is not just to steal or vandalize, but to use instability as a smokescreen; a distraction the provides opportunities to harm others.

Psychopaths like to exploit the chaos of political turmoil to indulge their violent tendencies, or to convince others to do the same.  If no one acts to eliminate the first wave of criminal actions during a social breakdown, then thousands of other criminals will also move to take advantage.  The first wave becomes an avalanche, all because the system no longer provides sufficient incentives to behave.