If you apply this rule strictly, then the mobs could first hit the police. In many countries, police officers are violent and have a de facto impunity when they break the law.
In the US, the police kills more than 1000 citizens each year, especially black men and teenagers. They very rarely face jail, not even loosing their job.
In my country, France, over the past year several dozens of citizens have lost an eye because of police shooting, a few have lost a hand because of grenades. The justice system protects the policemen, even when the videos are explicit. And when a juge tries to do his work, he can't. A grandma was killed while closing her window because she received a grenade in her head. The commanding officer refused to let the juge expertise the grenade launchers of his squad. He was promoted soon after.
Most people that prone self-defense and rant about some bad guys' impunity do no wish to see this fact: most policemen can break the law and not face the consequences. They're the only category of citizens that are above the law.