First, just because the state loses its monopoly doesn't mean some other entity gains a monopoly. In Mexico, cartels aren't just fighting the state, they are also fighting each other. If the state collapsed, they will continue fighting each other, and the people of Mexico will effectively live in a semi-permanent warzone.
Second, not all violent entities are created equal. Cartel members are motivated by greed and lust for power. Many are willing to employ tactics such as torture and terrorism to advance those goals: https://www.sfgate.com/news/article/Mexican-drug-cartels-new...
I'm not claiming every cop is a saint, far from it. There is certainly room for improvement in the US at least. However, bad cops are newsworthy because we expect better from our cops than our gangsters. On a statistical basis, a much larger fraction of the police force is motivated by a desire to protect and serve the public. And our police forces are subject to at least nominal democratic accountability.
If due process requirements don't meaningfully differentiate cops from gangsters in your eyes, we might as well do away with them anyways.