Free speech is a protected right in this country and neither the left leaning radical groups nor the right leaning radical groups should ever resort to violence to either stop free speech or to protect their right to free speech. Violence is never necessary.
That said groups on both sides of the aisle sometimes gather in protest looking for violence. Obviously the Alt-Right, Nazis and KKK were looking to incite violence in Charlottesville and we see what happened. I also think some groups showed at the Boston rally looking for violence and it happened, in small amounts as compared to Charlottesville and was caused by either BLM or antifa. Don't know which.
The only way to stop white supremacy, racism, white nationalism, nazism, black trust of police, and far left antifa problems is educating our youth and children that these things are abhorent in our society. Its the only way to stamp out these extreme un-American values. Its not going to happen right away. It may take many generations to do it but the more we teach our kids about how bad these values are, the sooner it will eventually happen.