Violent gangs like this exist in our communities because we allow them to. We pretend that only the police have a responsibility to enforce safe living conditions and turn a blind eye to predators and thugs living among us. Much like governments, we get the neighborhoods we deserve.
I'm not going to go so far as to say we should leave the bodies of gangbangers hanging at the city gates for a warning, but tacit approval, either from fear of gangs or distrust of the justice system is allowing this kind of behavior.