One way to understand all this may be through Fourth Generation Warfare Theory. For centuries people have accepted that only The State has the Moral Authority to use violence. This premise is breaking down all across the globe in many, many different ways. The acceptance of violence in places like Baltimore and Chicago is just a manifestation of how the world is changing globally. Syria has had non state actors engaged in violence for years. Libya is another.
One could, it seems to me, reasonably predict increased acceptance of force used by tribes, gangs, and families in the future. In the next few months here in Virginia we could well see a serious blurring of the lines between just who is the legit authority when it comes to use of violence.
I think it is much more recent than that. It was 1918 when German Sociologist Max Weber defined a "state" as a human community where the government successfully claimed a monopoly on legitimate violence.
In the Anglosphere, there has always been the acknowledgement of the right to defense of self.
Natural Law conflicts with Weber on this.