It's definitely more complicated than this. A fundamental premise of enlightenment democracy is the establishment of a framework for the mediation of political power without the need for violence. So that ideal of nonviolence goes back much farther than both the US or the fourth estate, and it can be argued that it is actually a starting point for much of the modern world's political philosophy.

But in general, it doesn't take a ton of thought to imagine why cycles of political violence are unsustainable and unproductive. If violence becomes a primary form of political expression, then you will simply have every different group trying their hand. This is why we prescribe the state with a monopoly on violence - a principle even older than democracy.

That isn't to say that violence is never just. Ironically, contemporary existentialism tackles this issue pretty nicely by establishing some imperatives which revolve around the relationship between oppressor and oppressed. Primary among them is the acknowledgement that the most sustainable and desirable form of change is done through conversation with the oppressor (as in liberal democracy), and that anyone who rejects this imperative acts in bad faith, just as the oppressor does when they refuse to treat.

Simply put, to engage in violence is to ordain yourself the oppressor, and understanding the heavy implications of this action is critical to just violence. De Beauvoir argues that idealism is therefore one critical aspect of justice in all forms, as it seeks, by nature, to preserve transcendent humanity in others. And this is the ambiguity of the freedom fighter - the classic dialectical struggle will always reduce itself to mystification because ideals are not fixed like the flesh, against which violence acts. Therefore, while violence can be just, it cannot be justice, because it does not directly serve any ideal. As such, our morality must be "opposed to the totalitarian doctrines which raise up beyond man the mirage of Mankind" and "freedom can only be achieved through the freedom of others."