via historian Mark Naison:
The assassination of police officers is not only morally reprehensible, it has a history of undermining the legitimacy of non-violent mass protest movements against social injustice. No better example of this can be found than the Haymarket Affair of 1886, which took place in the midst of a nationwide mass protest movement for an eight-hour day. One tiny, but highly visible, component of this protest movement were anarchists, who claimed that the armed force of government would always be used against workers and urged that workers arm themselves against the power of government and use dynamite as their weapon of choice to neutralize police, the army and state militia. The anarchists numbered several thousand among a movement of millions, including the 600,000 member labor reform organization The Knights of Labor, yet one one fateful day, their influence proved to be deadly. A huge eight-hour day rally in Chicago was taking place, when some person or persons, threw live dynamite into a line of police who had arrived to break up the rally, killing more than 10 police officers.
The national wave of revulsion against this bomb attack proved so great that it completely destroyed the eight-hour movement, contributed to a precipitous decline in the membership of the Knights of Labor, and put organized labor on the defensive in the United States nearly a decade.
I am not saying this to suggest that history always repeats itself, but to warn that legitimate non-violent movements raising important issues can be undermined by immoral and irresponsible acts of violence launched by those who can be linked to the protests, even tangentially, by the authorities of the time.