vigilante: a member of a volunteer committee organized to suppress and punish crime summarily (as when the processes of law are viewed as inadequate); broadly: a self-appointed doer of justice.
Harry Magaña is clearly a vigilante in the broad sense of the word. His role raises interesting questions about the complete breakdown of the state apparatus for dispensing justice as portrayed in the novel. We grant a monopoly on violence to the state in connection with the social contract on the presumption that the state will effectively employ that violence to provide security for the citizens. Bad cops, for example, constitute a breach of this social contract. In Santa Teresa we witness a breakdown of the entire social contract. It should therefore be no surprise that vigilantism then comes into play to fill the breach.
We need to discuss this more, but in the meantime the observations in Daryl's entry at Infinite Zombies entitled “One More Po” and the comments appended make interesting reading. I continue to ponder that entry and those comments.