The time for violence is when you believe violence will achieve your goals better than peaceful methods.
If you believe that this attack is going to make the alt right more able to carry out their agenda, then yeah, I suppose you would support the left taking similar measures. But anyone with any skin in the game is trying to distance themselves from the attacks, because the politics of this are horrible, and so would, say, bombing or shooting up the rally before all this happened.
MLK didn't choose nonviolence because he was a starry-eyed idealist. He did it because he knew that power in America flows from public opinion, not from the barrel of a gun. That might change, at some point in the future, but right now we live in a liberal democracy that is at least partially responsive to the will of the public. Winning PR victories is vastly more important than killing a few grunts.