If history teaches us anything at all, it's that violence is sometimes justified, unfortunately, in the pursuit of justice and equality.  

And before you give me that song-and-dance about the non-violent successes of the Civil Rights Movement, you should note that, after Brown v. Board, the organized violence of the US government was on the side of civil rights more often than not.

You'd also need to face something that even Martin Luther King acknowledged: that the existence of Black Power groups and militants like Stokely Carmichael, Malcolm X, Huey Newton and others made his own brand of non-violent social struggle much more palatable to the power elites in DC and the segregated South. 

Sometimes, violence is the only thing that creates a political and social ground upon which social change can occur.  This is one reason why the violent posturing and rhetoric of the white supremos is so dangerous: it could alter how power addresses domestic disorder and conflict.