If white supremacists resort to violence too, they should be arrested as well.  There are laws and precedents as to what constitutes hate speech.    The law does provide some latitude for unpopular and even offensive speech, as long as it doesn't cross the line into hate speech. The courts can deal with this.

As for the  masked and weaponized alt-left counter-protesters, they are unwelcome and their violence should be prosecuted to the fullest extent of the law.  Let's rid our society of the blight of violence.