This is a really good article going over class basis of pacifism, and touching on its historical failure to undo injustice:

What about Pacifism / pacifist socialism? Is violence necessary to acheive socialism? What about Martin Luther King Jr. and Gandhi? audiobook

Some quotes:

Will the peaceful abolition of private property be possible?

It would be desirable if this could happen, and the communists would certainly be the last to oppose it. Communists know only too well that all conspiracies are not only useless, but even harmful. They know all too well that revolutions are not made intentionally and arbitrarily, but that, everywhere and always, they have been the necessary consequence of conditions which were wholly independent of the will and direction of individual parties and entire classes.

But they also see that the development of the proletariat in nearly all civilized countries has been violently suppressed, and that in this way the opponents of communism have been working toward a revolution with all their strength. If the oppressed proletariat is finally driven to revolution, then we communists will defend the interests of the proletarians with deeds as we now defend them with words.

• Engels - Principles of Communism

On the question of whether the armed struggle is the only path to liberation, I would answer that at least in the case of our country, we have no other path. And we think that in the immense majority of latin american countries, there is no other path than the armed struggle. It seems to be the same case for countries in Asia and Africa. In general imperialism counts on, in every way, joining forces with the oligarchy, of every country, to impede the democratic revolution in every country. And its hanging people with a rope that can only be cut by armed struggle.

Revolutionaries didn't choose armed struggle as the best path. Its the path the oppressors imposed on the people. So people only have two choices: To suffer, or to fight.

• Interview with Fidel Castro on the question of violence