The article talks about Peru, so for the moment, let's scope to that. According to the article, Peru's crimes tend not to be violent, yet the vigilantes can be. Some of the cases described appear to show a lack of restraint - consistent with mob mentality. Also, if the mob is wrong? Nobody cares.
Now let's extend this to places that are actually violent. Organised crime run the show. They could easily pay/manipulate these groups to do their dirty work. Anyone who threatens the organisation will have themself and their relatives killed, so the hardened criminals remain protected. Lawlessness still reigns supreme. Now, you may argue that it's difficult to track the orchestrator (which isn't hard). But, if it were, they make a statement by killing people randomly who may have contributed. Organised crime will not tolerate these vigilantes. Violence begets more violence.
What's also missing in your argument is peaceful protest. You assume violence is the only option. Think of Gandhi.
I'm not sure if your comment attacking moralistic people is a dig at political correctness. I support any constructive criticism of political correctness (and there are many valid criticisms), but morality? I'm assuming you've lost something in translation. The world you talk of and are willing to tolerate sounds like the antithesis of my world.