I think you miss the point.
No matter things went for the worse or not. The people who used violence to solve their problem solved it. Even if they were identified, and a world of trouble fell upon them, the problem is solved. Even if those gangs learned and grew stronger in other city, their problem is solved. No more gang activity reported in the city for several years? That must be a super-outcome from the vigilantes' point of view.
Examples of this can be found if you want to check them. A thief on the run with some megabucks is ran over by a police van -the theft is solved. Thieves of chickens are found in the village, get hit with sticks, leave the town, no more chickens disappear. The problem for chicken owners have been solved.
In fact, whether it creates more trouble or not, it does not matter for demonstrating violence is used precisely because it has problem solving capabilities.