A needed plot twist

I think it would be a good twist in one of those vigilante movies (like Death Wish) if the protagonist, instead of executing murderers, accidentally executed an innocent person due to a case of mistaken identity. Or he executes someone guilty of a non-violent petty crime. This is why we have civil rights and due process instead of just murdering people we think are bad. Even with due process, horrific errors can be and have been made and innocent people are railroaded, but at least there's a chance of exoneration in front of a court of law. That does not exist with a vigilante.