Mob justice is blind
Criminals such as robbers, muggers, looters, etc have had a free hand for a long time. Due to the police's inability and reluctance to take any action, the innocent have suffered greatly. The police may succeed (once in a while) in sending some notorious somebody to jail, but can they keep such persons off the streets? Not at all. These criminals spend a single night in jail, and before you know it they are back to their old ways again.
People have decided to take the matter in their own hands in order to deal out justice themselves. Superheroes don't exist in our society, so it's up to the common Joe to get things done. As a result, if criminals are caught nowadays, they are bloodied up, usually to a critical condition, then handed over to the police. Most of these people don't end up in jail... they usually end up in a hospital or morgue. People are sick and tired of being oppressed and now they are shouting out their dissent. Each lynched robber carries out the same message from the people: "You can run from the law, but you can't run from us." It is indeed a sadistic way of meting out justice, but it is the result of police failures.