Well, if I adopted that vigilant thinking way, I could probably think to myself that civilians are those people that are defenseless and that try to stay out of trouble and away from violence.
In real life, if you see a police man beating a criminal, you, as a civilian don't usually go and help the police... you let them do their jobs and you stay away to keep yourself out of trouble <- this is a typical civilian.
that said, me, as a vigilante, would probably think: "That "civilian" that is helping a corrupt policeman is not a civilian at all, he's just a troublemaker and a violent person if he's in such a worry to step in into battle... in that case, i should terminate him too"
but that's just one of possible outcomes... I guess situations like these, depend on the Vigilante in question and not on the Whole group of vigilantes... because everyone will have their different personalities and different ideas, and 2 vigilantes will probably deal with that situation in 2 different ways.