She killed in self defense
I guess you can consider her a vigilante since she carries a gun in her purse and doesn't back down from trouble. All the killings shown onscreen are justifiable self defense.
the guy in the truck points a gun at her while she's skipping down the sidewalk, she's justified in killing him rather than get abducted and presumably raped and/or killed.
the killer in the fast food, hot dog joint pointed a shot gun at her so she acts in self defense.
The child molesting priest, well, I guess that wasn't justified, that was actually murder vigilante style. She straight shot him when he posed no immediate threat.
The only time you can kill someone in self defense is when they pose an immediate physical threat to you or to another person. By immediate physical threat, I mean you have to believe they cause death or serious injury.